There is very little room in which to work changing the tensioner on these cars.

If you haven't done the job before It's actually easier to remove the radiator. This in itself is easy.
You need to remove the fan, fan clutch, pulleys for the water pump,power steering and fan. - Its easier if you loosen the bolts on these pulleys with the belt still on and tensioned.
Once all these parts are out you will see a Y bracket that keeps the tensioner in. Removing the tensioner is pretty straight forward from this point.

There is a 19mm bolt that tightens the tensioner - you will find it. Loosen it before making any movement with the tensioner lever.

Do a search on M103 tensioner - there are many posts on this.