I have a Username automatically for a site, but I put a G instead of g, and when I try and log in it won't let me change Godwill to godwill. How do I delete this log in?

I can not change my Username for a program I involved with. I have it set up that the Username and Password is automatically populated when I sign in, but I put Godwill and it should be godwill, it's case sensitive. How do I delete it. Once I type the letter g, Godwill will automatically populate in the username box.