Hello there, I wounder if I could replace my 5.5 heater with a 6kw heater and or how do I figure this out?

500 extra watts will not help you.

Perhaps you can explain what the real issue is?

The real issue is that I have a used 6kw heater and want to know if it will work, Same size everything but mine is a 5.5 and I have a 6kw that I want to keep around as a back up if I can use it.

If this is a digital system, no... no way I'd up to 6kw... unless it's source power fed by a separate contactor.

Otherwise, I don't see a problem, however you have to take into consideration overall power requirements (max load) of the spa and whether or not the extra .5kw is going to have an effect.