How to heat up already baked french bread in oven to get a crispy crust
In my experience you have 3 choices. Lightly spray with water, oil of your liking (olive, canola, grapeseed, or sunflower not vegetable, and lastly, my favorite, brush on lightly sweet unsalted butter.  Preheat your over to 350 degrees F (give or take 25 degrees) and do not keep in oven more than 5-7 minutes depending on how round your bread loaf is.  Another idea is to slice the bread, spread lightly with unsalted sweet butter and bake for 1-5 minutes in preheated oven 350 degrees F and keep a very close eye on anything you do with all these suggestions because you don't want to end up with croutons. Good Luck and realize I myself would bake my own suggestions starting with less time, let's say 1 minute first, check then add a minute, etc.