1- Yes, by all means, change the tensioner as well, most belt problems are caused by worn out tensioners, try to get a Gates tensioner from an auto parts store, the one that was in my car was a Gates tensioner with a MB logo and it was identical to the one that I bought from the local auto parts store. You will probably save a lot of money.
2- The old style has a male torx on the pulley, you need a E10 female socket, using a ratchet, rotate counter clockwise and insert a pin or a fat nail to preload the tensioner in the back, if you're not sure how to do this, buy a new tensioner and look in the back to see how it is done. Remove the belt then the 2 bolts holding the tensioner to the engine block.
3- Just follow 2 without removing the tensioner, release the tensioner by rotating it counter clockwise until you're able to remove the pin, then ease it in a clockwise direction.