Ideally, you would be using the same weight for each set - but sometimes you may find that you can increase the weight, that's fine! Grab a heavier weight and finish your set! And other times you may find that you need to drop the weight in order to finish the set. Remember, proper form is always more important than staying with a heavier weight. For example, if I were doing 4 x 12 shoulder press, I would grab 20 pound dumbbells, and I would do 4 sets of 12 for as long as I could, but I may need to drop to 15's for the last couple of reps. Hope that helps!
