If your intention is to replace ubuntu then reformat the drive to NTFS with gparted.
If your intention is to dual boot windows and Ubuntu then shrink the ubuntu partition to make room for windows and go ahead and format the empty space as NTFS. If this is your plan then bear in mind windows is going to erase your Ubuntu bootloader. No big problem. After the windows install is done boot to your ubuntu install medium and install boot-repair https://help.ubuntu.com/community/Boot-Repair
and it'll fix it right up with the default repair. You can also put boot-repair on it's own disk, handy to have around.