This really depends on how long the brownies have been out of the oven... If you remove the brownies from the oven and then immediately discover that they haven't baked long enough, simply put them back into the oven at the same temp and finish baking. Once the brownies have completely cooled, you can reheat the oven and put the brownies back in for about 1/2 the time that they should have stayed in the oven. (If it seems you took them out 10 minutes too early, then put them back in for 5). This will cook the center but will likely not puff the top or make other changes. If it were me, I would simply make these brownies into something else like a trifle or break them until into small pieces to use for ice cream and pudding toppings.