You simply open up the partition editor and delete the partition, then preferably resize your ubuntu partition to make use of the remaining space. Though from personal experience, it was a pain in the ass to get my windows back on when I decided I wanted to use it for something again after I had deleted it, so I recommend that maybe you just keep it as a very limited partition. I'm sure you can spare a couple gigabytes for windows. Though if you are pretty experienced at installing things and aren't worried about the trouble, then go ahead and delete it by all means.