Head to your discography page, view the track you want to move, and click the Edit button beneath the title.
In the track editor, check the "part of an album, EP, what have you..." drop down, select the album that you want to move it into (please note: the album must be published). This will move the existing track into the new album, it won’t duplicate the track.
When you’ve selected the right album, don’t forget to click "Add to Album"! Voilà.