Unfortunately the answer is no, it is not possible to rename an individual homework problem. The number was used as a primary key in the database at the beginning of time (e.g about 1999) and so is not easily changed without breaking things. Set names are also primary keys, so once they are set you can't change them. (You can however edit the pop up dialog that occurs when you hover over the set name.)
This remains one of the high priority items to fix if/when we redesign the database. That, however, is a big job -- particularly since backward compatibility will need to be maintained.