The forum almost daily gets people asking how to change their username, and helpful forum users keep spending time to tell them it’s not possible.
My main suggestion is, if it’s possible with the Fiverr site at all, to either offer it (for a fee, so people won’t want to change their username on a weekly basis) or, if technically possible at all (I know sites that have this, so in theory at least, it is) to introduce the option of a display name, so that the username is only used internally, to not break content, but the display name is what people actually see (would need to be unique too, obviously).
If it’s not possible or wanted at all, it would be good to clearly make people aware that the username you choose is not just a backend but your frontend name too and that you won’t be able to ever change it. At least then, new people signing up won’t be stuck with a name they didn’t think was the name their customers will see, that has typos, etc. (a big and clear notification would make many think and check twice most probably).