It depends on your lease and local laws. If your lease forbids tenant's from changing the lock, you could just hire a locksmith to change the locks again. If your lease doesn't say anything, but does have a clause about no alterations to the property, you could also hire a locksmith.
BUT.. you really must check your local laws, eg: in Mass. if not forbidden in the lease, the tenant has the legal right to change the locks and not give the landlord a key.
