Change from UEFI to Legacy

I advise against doing what you're asking to do, particularly in light of your comment in response to the question of why you want to make this change:

Just don't care about it.

If you don't care about it, then the saying "if it ain't broke, don't fix it" applies. Absent a compelling reason to do so, switching from EFI-mode to BIOS-mode booting offers no benefits, but poses several drawbacks, including:

• Risk -- Such a conversion runs the risk of creating new problems you're not experiencing now. These risks include damage to installed files and the possibility that you'll end up with an unbootable (or even completely trashed) system.
• Time -- It will take time to perform such a conversion. You'd probably be better off doing something else.
• Slower boot -- Although this isn't universally true, EFI-mode booting is usually slightly faster than BIOS-mode booting.
• Reduced functionality -- You might not care about it, but booting in EFI mode provides you with some minor features that BIOS-mode booting does not provide. Most importantly, you can install multiple boot loaders and change which one is active with a single command. EFI boot variables and access to EFI system calls provide the potential to do more, although these features have yet to provide significant benefits in Ubuntu.
• Lack of Secure Boot -- This much-maligned feature actually provides some modest benefits because it can prevent pre-boot malware from running. To be sure, not all EFI-based computers support Secure Boot, but just about any desktop or laptop computer that shipped with Windows 8 or later does.
• Complications to current and future multi-boot configurations -- If you're currently multi-booting with other OS(es), switching from EFI-mode to BIOS-mode booting requires converting all your OSes. (Well, there are ways to boot different OSes in different modes, but they're tedious.) If you're currently booting only Ubuntu, enabling the Compatibility Support Module (CSM) in the firmware, which is required to boot in BIOS mode, makes it more likely that you'll run into problems when installing new OSes. See my page on this topic for all the gory details.

To be sure, there are scenarios in which switching to BIOS-mode booting makes sense. You, however, have provided no such compelling reason; you "just don't care about it," to repeat your words. Given the preceding list of drawbacks to such a conversion, "just don't care about it" does not qualify as a compelling reason to take the risks and accept the drawbacks of switching to BIOS-mode booting.

If you insist on ignoring my advice, you can do the conversion. In brief, you must enable your computer's CSM and install a BIOS-mode boot loader for Linux. The most common of these is the BIOS-mode version of GRUB. The Boot Repair tool will do this in a semi-automated way; however, you must boot the OS used to run Boot Repair in BIOS/CSM/legacy mode, and this in turn requires either luck or that you understand how to control the boot mode. See my page on the CSM, referenced earlier, for information on this topic. If you use GRUB, you should also create a BIOS Boot Partition on your disk (it probably does not exist now).