To know what new rent should be applied in a rental contract once any of the corresponding annuities has ended, the update rates agreed by the parties can be used, but the two most common are the Consumer Price Index (I.P.C.) and the Competitiveness Guarantee Index (I.G.C.), prepared and published by the National Institute of Statistics (I.N.E.).
The easiest way is to use the calculation program made available by I.N.E: