If your exhaust leak is located around the ball and socket area of the flex joint, replacement is the only effective option. You can attempt to tighten those bolts, but typically the bolts are rusted and will not turn and could break off at the head. If the leak is located somewhere other than the ball-and-socket area, you can try using products made to repair exhaust leaks, sold at your auto parts stores.
Hope this helps!
A. J. Valle