When blood sugar is low from exercising what do I need to do to bring it back up?

Any simple sugar. Will bring up your blood sugar quite rapidly. Examples of foods containing simple sugars are things like fruits/fruit juices(which is why diabetic often reach for orange juice when their blood sugar is low), or honey. Remember if you are not a diabetic, exercise induced/post exercise hypoglycemia(low blood sugar) isn't that common.
Eat. it is as simple as that blood sugar responds to food. Eat complex carbs or proteins before or during your workout. You could also drink them.