Change WordPress Log-In Username?

I’ve been hacked twice recently and each time the hackers change my WordPress username.

How do I change it back?
Next to the usernames box, it says ‘Usernames cannot be changed’ and I can’t type in the box.

Like this: https://i52.tinypic.com/sfkkmv.png

But obviously, there has to be a way that it can be changed, if the hackers are changing it.