At this time we don't offer an option to change usernames.
If you want to upload videos under a different name you will have to create a new account.
Alternative, you can create a channel within your existing HERE:
The Name you choose for your channel will determine your URL, f you name your channel "MyRumbleChannel" your channel's link will be ""
The Title is what other users will see in your banner; you can choose "My AWESOME RUMBLE Channel" if you want.