(This answer borrowed verbatim from
here)
Run a cmd.exe process with administrator privileges
Run diskpart
Type: list disk then sel disk X where X is the drive your boot files reside on
Type list vol to see all partitions (volumes) on the disk (the EFI volume will be formatted in FAT, others will be NTFS)
Select the EFI volume by typing: sel vol Y where Y is the SYSTEM volume (this is almost always the EFI partition)
For convenience, assign a drive letter by typing: assign letter=Z: where Z is a free (unused) drive letter
Type exit to leave disk part
While still in the cmd prompt, type: Z: and hit enter, where Z was the drive letter you just created.
Type dir to list directories on this mounted EFI partition
If you are in the right place, you should see a directory called EFI
Type cd EFI and then dir to list the child directories inside EFI
Type rmdir /S ubuntu to delete the ubuntu boot directory
