Before you remove the belt look closely at the tensioner pulley where the belt makes contact. If the belt is not centered on it then you need to replace the worn parts of the tensioner. Also, with the belt off spin the idler and tensioner pulleys. If any of the pulleys spin REAL easy (or real hard) replace em. Typically, these items on MB's can run out to 200k miles before needing to be replaced but it is a good preventive maintenance type measure. Also, see if the ps pump, alternator or a/c pulley are "harder to spin" that you think it should. That could be root cause of squeek if the squeek just recently came up.

As already stated, the belt can be changed out but is somewhat of a pia without removing anything other than slacking off tensioner.

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Jim