Correct mounting of a mercury bulb thermostat is critical to it's accuracy. Dust can also affect performance. Mechanical thermostats are usually adjustable, but require special tools. Most electronic thermostats are programmable. They use a thermistor or special sensor to determine room temperature and circuitry to switch the heating or cooling on and off.
Some electronic thermostats are adjustable.
The installation instructions will either explain the procedure or offer a phone number for technical help. Batteries can be used to power the thermostat or be used to retain programming in case of power outages. Before removing or replacing a thermostat, determine the type of power supplied to it. Most homes with electric baseboard or wall cavity electric heat have thermostats that are fed with line voltage. Power must be turned off first to avoid injury.