This is a really big job, you cannot just convert the drive from Command Prompt form inside Windows, here are the basic steps and it is not guaranteed to solve the problem, because your BIOS is so old..
1
You would need to create a new bootable Windows 10 USB With Rufus, and set that to MBR when creating the USB.
2
Backup your files.
3
Boot your PC from the USB and use these steps to convert your drive to MBR before you install Windows.
At the very start of the installation process, press Shift + F10, or Shift + Fn + F10 to open Command Prompt
Run this command and hit Enter:
diskpart
Run this command and hit Enter:
list disk
(Make note of the disk number you want to convert and enter it into the next command - replace X)
Run this command and hit Enter:
select disk X
Run this command and hit Enter:
clean
Run this command and hit Enter:
convert mbr
Then close Command Prompt
4
Then when Windows is installed, in BIOS set it to boot in Legacy BIOS