The number indicates the length, I'd say it's important to get the right one. Order from your dealership.
As for replacing them, I pulled my fan shroud out and removed the fan, makes it a lot easier, but I'm told it can be done without doing so. The 2 longer belts are held by tensioners, the bolt through each tensioner hold everything in place, and a bolt leading down to the tensioner is what adjusts them. The alternator belt has the locking bolt and adjusting bolt on its bracket.