How do I handle the deposit when replacing 1 of 3 tenants?

[Corbin H.]

I rented my first property starting in February of 2014 and we are coming up on the end of the lease.  1 of the tenants has notified me that he is going to move out at the end of the lease, but the other 2 want to stay.  They are currently looking for a new person to take his spot, and it looks like they are going to fill out an application.  

My question is, if this person is acceptable and we let him move in and have the 3 sign another year lease, how do we handle the deposit for the tenant moving out and the one moving in?  This is probably a very basic question, but I am new to this so any help is appreciated.