Which username is it that you need to reset? For example:

• A website username?
• A form entry on a website that has an incorrect suggestion?
• Your Firefox "Sync" username?
• Your main Android device account username?

A user name for an android device

I'm not sure where that is set. I'm familiar with your Google account (go into Settings, then Accounts), and you also can set a PIN to unlock the device (go into Settings, then Security). Is the user name you want to change either of those things? If not, could you provide a little more information about when you enter it?