new owner - what to do about getting new leases

So recently purchased a 6 family building. 3 tenants have existing leases and 3 are MTM. What is standard operating procedure in getting new leases with them. Are current leases with old owner null and void? Can I immediately raise rents? I do not want to "rock the boat" but just want to know what I can and cannot do and to be aware of anything that has certain restrictions.