Open Marvel Rivals
Go to the main menu
Open Settings
Find the Account or User Center option
Sign out of the current account
Close and relaunch the game if needed
Sign in with the other account
If the game is linked to a platform account, switch the platform login first
If you use a launcher like Steam or Epic, log out of that launcher and log into the other account
If account switching is not available in-game, contact support for account access help