The property is RENTED, meaning you are paying somebody else to have a roof over your head. But that somebody else still 100% owns that property. Changing the locks is something you should inform them of and provide them with the new key to THEIR property. Or you could be recieving a lovely eviction notice, which is perfectly legal for them to do.
See that word? Legal? Just because you can doesn't mean you should.
Curious as to why you feel you need to change the locks anyway. As it is definately a need to know thing for both landlord and whichever company he pays to manage the letting. As others have said, check the small print of the contract, it may say you can't change locks. If it does, more the fool you for not reading fully or chosing to ignore a Legally ( see it again) binding contract.