You can do the bottom pulley while you're at it. But they can be very hard to get off without a windy gun.

You'll also either need to get one of or fabricate your own device to release the tension. Even with the tool it can be hard to get the belt back on. It's easiest to have one person on the tool, and one prising the belt back on.

Also, note very carefully how the belt goes, as you have to take one of the brackets that the belt feeds behind, and it's a ball ache if (like me) you refit the bracket, before running the belt correctly.