When the concrete was poured an eye bolt should have been inserted for fastening base plate to. If this porch old which it probably is seeing that you are replacing it you can get a concrete drill bit, drill a hole, put in insert into the hole and the fasten the base plate to that. If the post were fully submerged into the concrete base. It will just rot out. If this is the case you'll have to remove the concrete post and pour a new one. But don't worry that sounds harder then it really is. Home Improvement books will show you the way.