i need to install windows 8.1 with Ubuntu (dual boot) but i can't because there's only one partition so i need to remove Ubuntu to delete the existing partition and make new ones.
If you have enough free space in your only partition, you can "cut out" another partition from it. You can use gparted for that. Select your partition, reduce its size by the amount you want the new partition to have. Then you have that removed space as free unused space. Create the new partition from it and install Windows there. I had done this a lot of times without any issue, but anyway I would recommend you to backup your disk first.
If you don't want to do this, you can remove everything and create all from scratch. You can do a backup of your programs and their settings with Aptik.
sudo apt-add-repository -y ppa:teejee2008/ppa sudo apt-get update sudo apt-get install aptik