Shut off power,unhook heater, turn spa on,if breaker trips its possible its not the heater but something else.
If breaker doesnt trip run spa for 20/30 min see if breaker trips.if it doesnt, shut power off hook heater back up and turn on the spa,if spa trips pretty much right away you can start by replacing the heater,but it could still be something else.
This is more or less just a quick way to tell if the heater is the problem,there still could be something else going on,but 80% of the time its the heater.
Or call a service tech and get it fixed the right way.