Ever synced before? In iTunes, when your iPad is connected, do a backup and sync. Check the button/tabs along the top of the window. There'll be one for movies. You can designate what to sync. Turn the movie you don't want, off. It will store the movie. Sync. Presto, it's gone from your iPad.
As to getting it onto Dropbox, if you were thinking you would need to store it somewhere, well, it's now stored on your Mac. If you need for some reason, to move the movie to Dropbox, well, that's. Mac OS file system stuff. It can be done.