Summer !!!!

So excited... we had snow for soooooooo long that we didn't get much of spring around here.  In fact, I think we had one whole day of spring.  It was like Pooof!  Spring.... and then summer started.  Ended up leaving for San Diego just as summer started here.  The weather in San Diego was nice and had warm afternoons and cool breezes from the ocean in the morning and evening.  But, nothing compares to the wild temperature ranges in Colorado.  This week has certainly been SUMMER!!! Total sleeveless and shorts and barefoot summer!  Loving it but.... now can someone tell me how to turn the heat thermostat to A/C?  It's like 74 degrees in the house and that's nice but a little A/C would be nice in the afternoon!  Installed the new thermostat this winter and it has saved us a bunch... but just realized that we have no idea how to switch it over.  Just another sweaty learning curve!