Windows 7, 8, & 10 all have a boot menu -- when you start a device from the off state [not hibernation or sleep] the menu gives you a choice of which OS to start, as well as several recovery or troubleshooting options, e.g. Safe Mode. The menu can be set to activate the default choice immediately, so you don't see that menu, and that's the norm with a single copy of Windows installed. Windows setup can add extra entries or choices however, and that can cause the boot menu to display. When you upgrade to a newer version of Win10, initially you can roll back to the earlier version, and that rollback can appear as an extra choice on the boot menu that disappears if/when you run Disk Cleanup to remove the old copy. I've also had it a few times add a choice based on the drive listing in the BIOs. If you've only got one copy of Win10 installed, and haven't installed an option to boot to WinPE for backup or Anti Virus software,the easiest fix to a newly displaying boot menu is to reset its timeout to zero, so it automatically starts Windows. The easiest way to do that is with EasyBCD, where you can also backup & restore your boot menu, and do things like switch between a Win7 style menu and the graphics based menu that became available with Win8.
neosmart[.]net/EasyBCD/
If you want to actually edit the boot menu and the BCD files & registry hive it depends on, EasyBCD may or may not do the trick. Windows boot menu, and the tools you use to work with it, evolve with new versions of Win10. The BCD used by a copy of Win10 may be pretty simple, recording basically the drive where Windows is located, and not much else, or it can have several added entries, including some referencing the firmware or BIOS. EasyBCD can only deal with the simpler stuff, so while you may be able to edit the boot menu itself, the results may or may not work.
I can't offer any guidelines on what will or will not work with EasyBCD -- 1.5 years ago when I added a NVMe drive, that added info in the BCD was required, and I had to use a Paragon HDM 16 Advanced boot stick to fix it. Since then I added a 2nd NVMe drive, because a regular SSD I was using was on borrowed time. Well that time was finally up last week, so I moved the copy of Win10 from the SSD to that NVMe drive. This time EasyBCD with it's simple entries worked -- before it clearly would not. I can't say whether new BIOS or new versions of Win10 are responsible. I used Macrium Reflect to restore a backup of that SSD to the NVMe drive, so I had to fix the boot menu afterward -- if I had used Paragon software instead it would have altered the boot menu for me, so there's a chance that if you use Paragon in that situation it might save time, but there's also a chance that Paragon's software will get that BCD modification wrong.
While there's no bulletproof method, you can do something like boot to the bootable USB stick for Macrium or Paragon or whatever backup software you used, and restore the boot partition with the BCD on it if Windows will not start after trying to edit the boot menu. [Note that the boot partition normally does not have a drive letter, may be hidden &/or restricted, & in this copy of Win10 2004, shows up in Control Panel -> Admin Tools -> Computer Management, as Healthy (EFI System Partition) at 102MB. The size is the giveaway -- the Recovery Partition is most always around 500 MB, the ~16 MB system partition is too small, and it should be clear which partition Windows is on.] EasyBCD will also run portably, if you have something like a Win2Go drive, or you can use Windows tools with a bootable USB stick with WinPE on it. I'd still bet on restoring the boot partition however -- Win10 2004 isn't finished, & the boot menu tools in Win10 1909 are broken.
There are two Windows command line or prompt tools you'll use working with the BCD... BCDBoot, to add a set of boot files, & BCDEdit, to change them. The simpler stuff with BCDEdit isn't fully documented on Microsoft's site, so it's best to use the built in help queries, adding /?. Back to the case or reason for this post, removing an extra boot menu entry... Use EasyBCD first, make sure that View Settings is selected on the left, then click/select the radio button for Detailed (Debug Mode). Every boot menu entry or choice should be listed. TO find the listing for a choice based on the firmware I looked for the Windows Boot Loader entry with no associated hard drive letter [this choice was not shown or listed in EasyBCD under Edit Boot Menu]. Then I selected & copied the ID, which should look something like this: {64e5266a-08a1-11ea-976f-8285fbc8030f}. To get rid of that choice I opened the Command Prompt, running as admin, and typed BCDEdit /Delete {64e5266a-08a1-11ea-976f-8285fbc8030f} /Cleanup , & pressed Enter. Done.