This part is not extremely difficult, but you do not want to end up deleting the wrong partition, especially if you have multiple partitions with important data on them that you would not like removed. From this point, you must be absolutely sure that you want to delete your system partition and reformat the drive. There is no turning back from that point, and all the data on the system drive will be deleted. Select the partition that your operating system is installed on, and press D to eliminate it. Now, Windows setup will ask you to confirm that you want to delete it. This is your last chance to turn back before deleting the partition. If you still want to proceed, press L to confirm that you want to delete this partition.
Windows will take you back to the screen where you selected the partitions. Notice that the partition you deleted now is unallocated space. Select this space and press C on your keyboard to create a new partition. Unless you want to create other partitions on that space, you should just let Windows use all of the space possible on it, so do not modify the allocation Windows has set for it. After seeing this screen, press Return/Enter. Now you will be taken back to the main partition screen.