1. Which cells it takes
Most thermostats take two AA cells, some take AAA, and smart thermostats usually take two AA lithium cells. The type is printed on the back or on the battery door.
2. Lithium is usually better
A lithium AA holds its charge for years, and a heating controller draws hardly any current. Alkaline cells in a smart thermostat tend to need changing every year or two.
3. Watch for the warning
A low-battery message on the display, or a screen that dims and blanks, is the first sign. Change the cells before the heating stops, because a controller with no power can lose its schedule and settings.
4. Change them together
Fit two new cells of the same type, in the same direction, at the same time. A new cell next to an old one can make the controller think the battery is flat.
5. Put the schedule back
A thermostat that loses power often forgets its schedule. Note the settings before you remove the cells, and put them back in afterwards if the display comes up empty.