IWe have a Chelsee hot tub from 2010. The spa temperature is set to and reading 90 degrees on the display but our thermometer says 115 and the water is very hot. What can we do?

IWe have a Chelsee hot tub from 2010. The spa temperature is set to and reading 90 degrees on the display but our thermometer says 115 and the water is very hot. What can we do?