To loosen the tensioner you need ideally to remove the fan shroud and viscous clutch and fan.
There is a 19mm bolt located between the water pump and power steering pump pulley. This must be loosened. After loosening use a 13mm socket and extension to loosen the tensioner arm. You can then remove the serpentine belt.
You may need to remove the tensioner completely to remove the bottom front water pump bolt. You will also be required to loosen the power steering pump to get access to the rear water pump bolts.
The top front water pump bolt has a allen key head - as in your diagram.

I removed the radiator from my 190e 2.6 to facilitate access. As you realise there is not a great amount of room at the front between the engine and radiator. You can also remove the distributor cap - it makes the job easier also.

There is a DIY sticky on this job in the DIY section. It relates to a w124 but basically is the same job.

Here is a picture of my 260e half way through replacing the water pump.
Note what I removed to facilitate the job. Other more experienced hands don't need to remove this amount of equipment however for the shade tree mechanic it makes the job easier. Note that this is a w124 so more room and i didn't remove the radiator.