Can't login into backend but have FTP access

I've once have changed my "admin" to another word "RM_Master" for security reasons. This was proposed by a plugin, and I think, that I've used this plugin to change the "admin"-name.
But this somehow seems not to work and now I can't get into my backend anymore.
I have FTP-Access to the site. Is there a way to figure out what went wrong and maybe to change it back or figure out how to login into the backend?