How do I chance my username on a Wiki?

Well, I hate my current username on the wikis I'm on, so I'd really like to change it. Not on Wikipedia, on a Wiki. Such as Bandipedia, Dragon Ball Wiki, Bulbapedia, and wikis like those. I was once told that I could change my username on wikis, but I checked it up and nothing came up. If you know how, please inform me of what I have to do, how long it will take, who I need to ask, etc. Thanks!