I did my 156 v6 timing belt and water pump this time last year,setting the tension was fairly easy as Ralf says,line up the pointer and the notch to set the tension,but getting to that stage in the proceedings is a lot of work and if you haven't got the right tools or confidence, forget it.
The water pump impellor is plastic and cracks,allowing the impellor to slip on the shaft so upgrading it to a metal impellor is a must if your changing the belt anyway.
One thing to bear in mind is that your car is ten years old,age affects belts as well as milage,so if the belt has only been changed once in ten years then it definately will need done again.
I would definately get it changed or sell it on for a newer 156,a decision that you should make fairly quickly because the risk of belt breakage will,due to the cost of repairs,effectively write your car off.
I had quite a nice 1998 156 2.5v6 this time last year with 58,000 miles on it,I paid £1600 for it,I spent I think £1300 pounds on parts alone(all my own labour) and it still needed more,I quickly made a decision to p/ex it early this year to get shot of it.
It was one of those decisions I made with my head not my heart
but someone got a brilliant 156 after I was finished with it.
If you are planning keeping the car then pay someone to change the belt , but be prepared to spend more money.