Unable to set user and password for more wpmu plug-in

I have a client who has a delvelopr who uses wpmu on their site.

The person is no longer working with them and their user name and password for wpmu main plug-in no longer work.

When I enter mine to rekey the install and work on the site it gives me a login error and the following.

Still showing the old user name…. How do I clear that out?!!

Love & Light GLP