Convert a Windows 10 MBR (BIOS) install to GPT (UEFI)
These commands may be carried out from a running Windows 10 installation or from a PE environment.
From a running Windows 10 installation, launch Disk Management from the start menu and determine your Disk number for your MBR Windows 10 installation.
Launch powershell as administrator from the start menu and run the following command (replace 0 with your Disk number):
mbr2gpt /convert /disk:0 /allowfullOS
This will convert the drive to GPT and make an EFI partition on the drive as well.
From a PE environment, you can use the following command
mbr2gpt /convert /disk:0
If you need to add your Windows 10 EFI Boot Manager to Grub, check out this page: http://ghorr.org/?page_id=641