Understanding that Ubuntu is already installed in the computer and if you don't wish to keep both Ubuntu and Windows (Dual Boot) Simply start the Windows Installation and format the whole disk in order to install.
Windows will list the partitions as "Unrecognized" and allow you to remove them if unwanted. After which you can choose whether to partition or not the hard disk drive in order to install. If you choose not to partition the disk, the whole disk will be used for the Windows Installation.
Make sure to backup whatever data you already have in Ubuntu before to remove the partitions.