nest Thermostat E Guide D'installation page 5

1. Switch off power
This protects you and avoids blowing a fuse in your
equipment. To make sure the power is off, change
the temperature with your current thermostat
and wait at least five minutes to be sure that your
system doesn't turn on.
or
Breaker box System switch
2. Remove the cover
Some thermostat covers pop off, while others
need to be unscrewed. Older thermostats may
have a second plate that should also be removed
to expose the wires.