Here is the manual with instructionsFrom the manual:
For 240 VAC Heater Operation:
Move the red wire on the main terminal
strip (TB1) from position #1 to position #3. Make certain wires are connected exactly as shown in Figure-D (Page 12) before applying power. Failure to do so will result in damage to the circuit board and/or related components and void the manufacturer’s warranty.
In the 15A/30A configuration, the heater will not operate
at the same time as the high-speed jets pump.
The factory setting is 120V/15A.
**
If the spa is to be operated on 40A service, remove the jumper JP1
#1-2 on the circuit board to allow the heater to operate at the same
time as the high-speed jets pump (Page 38).