Replacing a thermostat most times is pretty simple. Thermostats run on 24V low voltage, so there isn’t any chance of electrocution, although you can get a jolt if you don’t turn the power off. Replacing a Thermostat is a good Do it Yourself (DIY) project if you are mechanically inclined or a handyman can do it. For more complicated heating and cooling jobs: new installation, recharging freon, replacing a compressor, etc I recommend calling an HVAC contractor, but for simple things like replacing the air filters, or replacing a thermostat- a handyman is fine. With that said, calling an HVAC company is always fine if you don’t want to deal with it, they just might be a little more expensive.