[iMore.com]

Snapchat may not allow you to change your actual username, but it does let people change their display names.
If you use Snapchat regularly and have lots of friends, display names can quickly become a mess. While some people use their regular names like good social citizens, others feel the need to change them to non-descriptive nicknames that can create confusion over who is actually sending you a snap. If you prefer order in your Snapchat universe, you can easily override their chosen display name in favor of one of your own creation. Here's how!

Full story from the iMore Blog...