Your Rocket League Sideswipe in-game name is tied to your Epic Games account’s display name. Your in-game name will still be pulled from your Epic account if you logged in with another platform.
Follow the steps below to change your display name:
Psyonix Support is not able to manually change or modify Epic Games names. If you run into any bumps while changing your display name, please contact Epic Games support.
Let us know how we can help you.