Transferring from Xbox series one to series x/s game on the same console

Hi, my son downloaded the Xbox series one version to his Xbox series s console and he’s realised this means he can’t play against friends. I’ve read about the dual entitlement but that talks about when someone gets a new console. How do we upgrade the game on the same console without buying the whole thing again? I’d be grateful for simple instructions as I’m old and my son is impatient, ha ha!