Right of Entry
A tenant is generally not permitted to change the locks in his rental unit as this violates his landlord's limited right of entry. The landlord does not have full legal possession of his rental apartment while a tenant lives in it, but he does have the right to enter for specific situations specified by the lease agreement and the landlord-tenant laws in his state.
Landlords commonly replace or rekey the locks after the previous tenants vacate. If the current tenants are uncertain of whether landlord actually did this, asking the landlord if you can replace the locks yourself may allow you to put your own locks in. Get extra copies of the keys to provide to the landlord if you are permitted to go this route.
If a lock is damaged or broken beyond repair, lock replacement is needed to ensure the safety of the tenant and security of the property. If the tenant or one of his guests damaged the lock, the tenant is responsible for the cost of the lock. Otherwise, the landlord is responsible for fixing the problem lock and giving keys to the tenant.
A landlord is not permitted to change the locks without suing for possession of his rental unit, also known as an eviction order. If the landlord does change the locks during the eviction process, but before the eviction order is given, the process is called a self-help eviction and the tenant can sue for damages.