I have an easy way for you to do this, without having to touch your database at all. If you can upload a plugin and activate it, then you can change your username.
Simply go to the page below and download the plugin. Opt-in if you would like, but it's not required. If you do opt-in, however, you will be the first to hear about new plugins and scripts that I write, as well as get great discounts on future products I create.
Wordpress Admin Username Plugin
PS: I HIGHLY recommend that you delete the plugin from your plugins directory after using it. It's not required, but if somebody figures out how to access that file from outside wordpress, then they would know what username you're using and it would defeat the whole purpose of doing this in the first place.
Please send me any feedback about the plugin too.