In case you need to change the account (For example if you linked the wrong one), you must first unlink the first one.
Follow these steps to unlink your account:
- Go to twitch, click on your profile name in the top right corner
- Select settings
- Go to the connections tab
- Navigate to where you can see the link between your twitch account and Rocket League
- Click on Disconnect.
- Repeat the linking procedure with the correct account.