That spa was made to run on either 220 or 110, but is not convertible. Sorry.
You can do it, but you have to start by buying a new heater - you can buy the original type of system: square heater box and twin heater elements. Still available, BTW. You can also install the new type of heater, which has a Power Down Reset system. Kind of cool system, if you ask me.
But in any event, you would then need to rewire the relay which controls the heater, and you will have to run a 30Amp, 220Volt, GFI-protected circuit to run just the heater. The rest of the spa still runs on 110volt, just as it does now. It's a lot of work, and if done wrong, could be dangerous.
I would recommend that you simply keep a real good cover on the spa, set the temp and enjoy it.
HTH
