Since baby's new to food now, baby prob wun eat so much at a sitting. So scoop what you think you need using a clean spoon (maybe 1 tablespoon) and feed baby. You can warm it up if you want but it's not really necessary.
As for the rest of the food in the jar (which is uncontaminated), you can keep it in the fridge for up to 2 days (or as stated on the jar). Or you can freeze it in small portions in icecube trays and consume within 2 weeks.
You can add the puree to the cereal that baby's already eating or let baby eat the puree on its own.