change of amount during the contracted period

this year i decided on taking a vox UPS which is a contract for 12 months, keep in mind it a 12 months contract stating that this product will be this much for the duration on the contract and then out of nowhere they increase the amount of the contract during the cause of the contract. this is ridiculous because when i took the contract it said whatever amount for 12 months how do they just decide to change the amount. it is not even about the increased amount but that is a contract for a specific duration why do they change the amount during the contract