I want to change the working hours of employees contracts this month?

A contract is between two or more parties and this means you must get agreement. Be careful here because you could be in Breach of Contract and or possibly other Employment Laws like the Part Time Workers Regulations, The Working Time Agreement, or even Sex Discrimination. Make a call to an ETAS office near you first or your employees could claim constructive dismissal or worse and you could find yourself with litigation against you.