Who do I call if I won't be there tonight but by 6 pm

You can't check in until 8pm. You have to call the homeless hotline and see if they have room if not they will try to find another place for you

Call the 800 homeless line tell them you're a resident they will give you direct number or get it at front desk before you leave

Just call and tell them.

Let them know you will be late

As long as you have a job, that shouldn't be a problem.