Fish died in the tank: What do I have to do to prepare the tank for a new fish?

I had a betta fish in a 5 gallon tank, and he died. I was away, and so his body was in the tank for maybe a week. I had someone coming to feed the fish and she did not see him, and when I got home I found his body (so apparently he was dead for at least a few days).

I want to clean the tank to prepare for new fish:
I know I have to put new water in.
I know I have to replace the filter.
I know I have to replace the gravel.

How do I treat or clean the tank, decor and fake plants to prepare for new fish, so it’s a healthy environment?