Open up the editor and select the beatmap you want, then select "song setup" at the top of your screen. You are able to change the Title field of the beatmap on the new window that pops up.
Note that this will cause any ranked beatmaps to no longer be ranked and you will not be able to submit any scores on them. Any unranked beatmaps will ask for an update to revert the title back to what it was before. If you do set any scores on these maps, once you update they will be removed.