Hey got any spare USB laying around? I know a risky method to force BIOS downgrade through a bootable USB. This method can potentially brick your BIOS, so do this at your own risk.

1) Download Rufus and install it from https://rufus.ie/

2) Plug in the USB. If you have anything important from it, back it up to your hard drive or something.

3) Open up Rufus then select your USB.

4) Under "Boot Selection", select "FreeDOS". Check to make sure that "FAT32" is selected under "File System".

5) Leave everything else at default value and click Start at the bottom. This will format your USB to FAT32 and install FreeDOS on it.

6) Go to this website https://www.dell.com/support/home/us/en/19/product-support/product/inspiron-15-5559-laptop/drivers and download the last BIOS driver that worked best for you.

7) Rename the file's name into something less than 10 characters, like 160.exe for example. Then copy/paste the file into the USB.

Now shut down the laptop and plug in the charger. Don't eject the USB yet.

9) Now I want you to boot into BIOS by turning the power on and spam F2. While in BIOS set up, I want you to disable Secure Boot, PTT/TPP security and then enable Legacy Rom Option. I don't know how does your BIOS look like so try to figure those out on your own.

10) Save all changes, then exit BIOS. The laptop will reboot, but this time I want you to enter the Boot Menu by spamming F12 before the Dell logo appears.

11) Once you enter the Boot Menu, you should see your USB under the "Legacy" list. Select it.

12) Now you should see some interface that looks like barebone Command Prompt. Congratulations! You have made it into FreeDOS!

13) Type "dir". This command will list all the files you have on your USB.

14) Once you see the BIOS file, type in "xxxx.exe /forceit" (where xxxx is your BIOS's file name)

15) Proceed with the instructions. Good luck!

Did all that.

I was able to go to the update bios screen of from 1.7 to 1.3. But stuck at the final stage. The same old message shows up.

Dell should take the ownership of making a newer version of bios and release it to users because many people will realize a Dell laptop without a battery runs slow like a snail.  

In order to run at faster CPU speed and charging battery, users have to purchase a new battery and Dell ac adapter every year.  This is a guaranty of profit.

Really furstrated.

By the way, I did try to use RUFUS. But version 3.8 or 3.9 do not work.  But I can use version 2.6 to create a bootable usb.

In the end, I use the DELL(TM) DIAGNOSTIC DISTRIBUTION PACKAGE to create my bootable usb.

Thanks for your suggestion anyway.

For the past three days, I have tried to log-in to give you a update.  But I can not log-in my Dell Community account for some reason.  Maybe because I am too harsh to criticize this issue and have been temporarily blocked by Dell.

Good news though. Starting 4/1/2020, I found out the 5559 CPU speed is jump to 2.0ghz or above without a battery (I normally do not insert battery into the battery bank). The 5559 still has the BIOS version 1.7.0.  This has been consistently for the past three days.

There is no logical answer for explanation of this phenomenal.  I noticed one thing, when I turn on my laptop, the window 10 has a welcome screen greeting. The only logical explanation is Window 10 do a update the night before.  This update solved the CPU speed issue.  Whether is because Dell gives a fix to MS or MS fixed it by themself.

Good news for 5559 users.   BTW I have not check whether this fix works for charging issue.

Even following all these steps, it was not possible to downgrade, the same message appears "update of bios blocked due to an unsupported downgrade"

Yes.  You are 100% correct.  I had tried so many different ways.

However, the CPU slowdown and not charging battery seem to be resolved now.   The strange thing is, after 4/1/2020 after I reported this issue was been fixed, my 5559 went back to the snail mode again for a few days later.   It was back to normal speed of over 2.0 Ghz and allow to charge battery sometimes later on.

I was hoping some Tech Experts or computer experts will able to provide the explanation of how this was been fixed.  It seems to me this is an drive incompatibility issues between Intel drivers that meant to slowdown CPU speed to prevent overheat and Dell and MS utility drivers.

I had the same issue. What did it for me was to use bios recovery rollback rather than downgrade

Keep pressing Ctrl+esc and turn your system on and then plug the power cable .

This will lead to bios recovery where voila you can rollback the bios.

If this procedure does not apply to your laptop or PC just search for bios recovery options rather than downgrading because recovery treats the current bios as corrupt and does not follow it's instructions.

i thought dell allows bios to be downgraded without hassle?

there is even a bios downgrade option in bios menu

Yes, Dell older models allow users to do bios downgrade without any problem.   But newer models have bios guard installed (I  believe it is hot-wired in the motherboard).  It is a good feature because this way virus programs can not change the bios and cause damage.  My issue is really because Dell 5559 can not charge battery and without battery in the battery bay, it only runs at 0.29 GHZ. Like a snail.  It appears the problem is because incompatible drivers between Dell, Intel and Microsoft. 

Hope this answer your question.