1week ago my knee popped out went to ER doc placed knee back & took ex-rays when can I take the long brace off go swimming or exercise on it?

Now! You should start doing straight leg raises and quad exercises immediately keep working on icing and getting the swelling down. There are special knee braces to stabilize the patella and get you out of the big knee immobilizer and walking normally. Swimming is ok but avoid breaststroke kick.
Follow up. With an orthopedic surgeon. He will probably want to examine you before he clears you for exercise.