There are a couple of strategies for recooking an undercooked pie. The easiest is to cover the pie with aluminum foil, place it back in the oven and bake it at 425 to 450 F for around 12 minutes.
Another strategy, though it takes more time, is to remove the pie's top crust and scoop out the filling. Put the pie back in the oven and allow the bottom crust to cook further while simultaneously heating the pie filling on the stove. Once the bottom crust cooks all the way through, reinsert the filling and place the top crust back on the pie as a crumble. Alternately, cover the pie with whipped cream instead of crust.