I don't think there are replacement parts to fix these thermostats. Your only option may be a replacement of the thermostat.

Is the thermostat less than 10 years old? It it is then it should be covered by the Carrier 10 year parts warranty if the system was registered at the time of installation. If it was not registered then the warranty is only 5 years. Replacement should not be too difficult.

Thank you, Mike. Thermostat is 8 yrs old; I'll check on the warranty. I was hoping for info re: whether there's a way for us to open the cover to pop the button into position ourselves.