Your car has the old tensioner. The new ones started in 95, but you need the VIN to know exactly which tensioner was fitted in the car. My 95 C280 has the new tensioner.
This is what you do. You need to loosen the big allen bolt (8mm). If you can't keep the allen wrench straight I recommend purchasing an 8mm socket type allen. Once you break it loose you only need to turn it about one complete revolution. Second step is to loosen the tension by turning that long 13mm nut at the top (counterclockwise). Pay attention to the white arrow on top of the allen bolt. It should be moving as you loosen the tension. That white mark is very crucial when putting back the belt. You don't want to over tighten it. Make sure the mark is about 3/4 of the way when tightening. The old tensioners are prone to problems.