Documentation for JIRA 4.1. Documentation for other versions of JIRA is available too.
Currently JIRA does not have the ability to change usernames. Manually editing the database is not recommended due to the number of places the username is referenced (comments and filters for example).
The best option in this case is to edit the XML backup:
If the username is a common word (e.g. admin), you may replace text that is not relevant to the user. So please be aware of this issue when performing the replace.