Many of the older, non-touch-screen models of Honeywell thermostats come with the option to lock their covers. The thermostat's cover is locked in place via a small clip cover. This clip cover is usually installed after the thermostat is formatted and before it is mounted to the outlet box. Honeywell recommends that only certified professionals install and remove Honeywell thermostats. Any damage to the unit due to improper installation or removal may void the warranty.
Remove the thermostat from the outlet box. The thermostat is affixed to the outlet box with two small screws. Use an appropriately sized Phillips head screwdriver to remove them.
Turn the thermostat over so that the back is facing you. The small cover clip is located on the lower left hand side of the back of the thermostat's base (when the thermostat is right side up).
Remove the small screw holding the cover clip in place with an Allen wrench.
Pull off the cover clip with your fingers.