If the water is coming from the seam between the nozzle head that turns and the riser that remains stationary, you will need to replace the head, in my opinion. Purchase a new head and just unscrew the cap and rotor assembly from the new one and the one in the ground and swap them out; as the sprinkler body in the ground will be fine for years to come.
This is much quicker and less time consuming than replacing the entire sprinkler.

I have several head that leak where the top of the rotor connects to the bocy of the rotor. There is a gasket but no amount of tightening will stop the leak. Any ideas on what to put on the gasket to stop the leak

@David Johnson: Have you tried replacing just the gasket? If it is old, it may be that it is not performing optimally.
@Matt: It may be as simple as replacing the sprinkler nozzle or, as more_to_grow mentioned, replacing the sprinkler head. This is much more cost-effective than replacing the entire sprinkler.

Here is a link that might be useful: Sprinkler nozzles