At the moment, there is no option in Webuzo to change username. You will have to reinstall Webuzo in order to change username.
We strongly recommend you to take full backup from Webuzo panel -> Webuzo Backup page and then reinstall Webuzo on your server.
Also if you are using Shared Hosting, then Webuzo cannot be used on shared host servers since Webuzo is a single user panel meant to be used by a single entity.