Since Windows 10 (1703 build 15063 – Creator’s Update) Microsoft natively supports conversion from MBR to GPT via MBR2GPT.exe tool. If you want to proceed with conversion of OS disk and switch Bootable mode from BIOS to UEFI then follow these steps:

If you don’t have at least Windows 10 1703 or Windows Server 2019 already installed (where you can issue command directly from command line), you will need at least bootable drive with this version (or newer) and boot into WindowsPE recovery mode and start command line.

Tip: For this purpose I recommend to use MBR2GPT from recovery mode.

Identify OS disk number which contains MBR partition by following command:

diskpart.exe
list disk

Once you identified correct disk number (where Gpt column has no asterisk) validate if you can do conversion (X represents your disk number):

mbr2gpt.exe /validate /disk:X

If validation went well you can start with conversion:

mbr2gpt.exe /convert /disk:X /allowfullOS

Now you will need to go to BIOS and switch in bootable options from Other OS (BIOS) to UEFI.