Here's a long technical explanation.
The short version is: in Windows boot modes are tied to specific partition table styles. You were booting in BIOS mode, so your partition table uses MBR style. To boot in UEFI mode, you need a GPT partition table.
Recent versions of Windows come with a
mbr2gpt tool that will do the conversion without data loss given that some prerequisites are met. If they are not, the tool should fail gracefully.
After the conversion Windows will not be able to boot until you switch to UEFI boot mode. Make sure your backups are up to date before proceeding.
Open an administrator's command prompt: press Win, type
cmd and press Ctrl+Shift+Enter. Confirm the UAC dialog if it appears.
mbr2gpt /validate /allowFullOS Enter. Hopefully you'll see a "Validation completed successfully" message. That means that the prerequisites are met. Use
mbr2gpt /convert /allowFullOS Enter to proceed with the conversion. Then reboot into BIOS and change boot mode.
If the validation fails, you're out of luck. It's possible to convert such systems, but it's not straightforward and the procedure differs on case-by-case basis, so for a beginner I'd recommend reinstalling Windows instead. You may also have to manually erase the partition table in this case (this will destroy all your data). You can do this in the Windows installer by pressing Shift+F10, entering
diskpart Enter, then
select disk 0 Enter and