Disconnect the Frigidaire's power cord from the outlet.
Twist the screw counterclockwise in the metal ring that holds the flexible hose on the rear of the Frigidaire. Slide off the hose.
Take out the screws that secure the back cover to the sides, top and bottom of the cabinet. Remove the cover and set it out of the way. You'll find the thermostat near the bottom of the heating element case on the left edge. The thermostat is attached to two wires.
Pull gently on each wire to remove it from the thermostat's contacts. Remember which wire was connected to which contact.
Take out the two screws holding the mounting plate for the thermostat to the heater element. Pull off the plate and the thermostat, which is permanently attached to the plate. Take this part with you to a repair store to match with a new thermostat.
Screw on the new thermostat using the hardware removed in Step 5.
Slide the posts on the two wires onto the contacts for the new thermostat.
Screw on the cover plate to the back of the Frigidaire.
Slide on the hose to the round port at the bottom fo the Frigidaire and tighten the screw in the metal band around the hose to hold it on the Frigidaire.
Reconnect the power cord to the wall outlet.