Tenant is terminating the lease

[Jeffrey McKee]

Month 2.  My inherited tenant is terminating the lease early. Under the existing lease there is a tenant and an occupant.  The occupant is willing to take over the lease.  I want to terminate/void the old lease and start a new lease/contract with the occupant as a tenant proper.  Do I have to do anything official to terminate the old lease with the previous tenant?    

Under the contract the tenant still owes 4,500 in rent through the expiration date. Contract states that all unpaid rents which are payable during the remainder of the lease will be accelerated without notice or demand.   Am I legally allowed to charge him anything if I took the proper actions to mitigate significant losses? 

Main question is what do i need to do to properly terminate existing lease to start fresh with new tenant?