Usernames cannot be changed or altered in any way. Your username is permanent, and there isn't a way to change it to something else.
What you can do is create a new account, and then upload your images into that new account. You can then delete your old account. You will need a different email for the new account, as an email can only be used on one account at a time.
We do not have a way to move all of your images into the new account, and do not have a way to combine accounts. It is up to the user to download and upload their images to move them to a new account.