Command prompt will probably be in a menu somewhere as terminal. There is likely to be a gui utility of some description to do this but I can't tell you how to use that in so few words.
When you find a command prompt
sudo fdisk /dev/sda
Then
d
If it asks for a number start with 1 and repeat with the next number in the range it suggests until it stops asking. Then type
w
(Hitting return where appropriate).
Done. Reboot. Install Windows.