At he website of the Ministry of Economic affairs you can get the index numbers for rent contracts.
For example. The rent started on March 1st 2002 and the rent was 1.000 euro. So The index number of February 2002 is 110,09. Then on March 1st 2003 we should have a new rent price. The index number for february 2003 is 111,56. We will mulitply the old rent price with the new index number and then divide by the old index number. The result of that calculation gives the new rent price. In this case it's 1.013,35 euro.