Thermostat read out changed to cel instead of faren. .. How do I change it back?

Some thermostats allow you to change the differential value - that is how far below the heating (or above the cooling) set point before calling for heat (or cooling in the case of air conditioning). The built in 1 degree differential setting provides the most even temperature in the space at the expense of more frequent furnace firing and stopping. Many people prefer this mode as opposed to allowing wider swings in temperature for less frequent furnace firing and stopping. If you wish to change this setting, you'll need to replace the thermostat as it is not user adjustable in this model.

Remove thermostat from wall, leaving base plate on wall.
Model number appears on back of device.
Google search thermostat model in following format: Honeywell xxxx32 thermostat .pdf
Adding .pdf to search parameter will yield product manual.
Also while thermostat is off the wall, change batteries located in compartment on back of thermostat. These batteries must be changed periodically, for example each 3-4 years.

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Change battery located on back of thermostat??

Go to google
Search for your model number 1234XX .pdf
Add .pdf in search parameter.
Honeywell posts most their thermostat manuals.
Open pdf file, and then Use Ctrl+F for search feature
Word-Search manual

To find model number on thermostat, remove from wall.
Model number appears on back.
Don't forget to replace battery while you have back exposed.
Dead battery can also cause lockout and malfunction

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Thermostats is in need of replacement and sounds like they are sticking. So it would be best to change both out. Turn power off to change them out it would be best for safety issues to change out as soon as you could. I have seen thermostats burn up in compartment on water heater.

Water heater can test for power, but is only receiving 120V instead of 240V.
Test that upper thermostat is getting 240V across top two screws of thermostat.

http://waterheatertimer.org/Test-electricity-to-water-heater.html

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I have the same model with the same problem. I think you will find that there are two temperature sensors which are located under the heater. (one at the bend and the other further along the heater. The one at the heater bend ie at the back right hand corner of the washer will be the problem. It is a non-resetting switch which trips out at 90c. To change it you need to remove the small metal panel at the back. You may need to remove screws holding the casing together to achieve this. ( Dont forget to switch the power off & remove lower tray)
Remove the wires from the two spade terminals. Put a multimeter across the terminals. If it is duff there will be no change in the meter reading.(If good the meter will read 0 - short cct.)
The thermostat unscrews from the housing - be careful because the housing might loosen instead and then you will have to put that back in . Pop in the replacement and refit the wires. Reassemble & test.

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1) Try changing both thermostats. Thermostats are frequently defective. Don't change 1, change both, using generic parts from hardware or home center or plumbing supply.
Be sure to put-back insulation and cover over thermostats so they are not exposed to cooler air and cause misread of tank temperature.
http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

2) Next step would be complete 30 minute troubleshoot, that would isolate the problem and give understanding of water heater operation.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

http://waterheatertimer.org/How-electric-water-heater-works.html

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Open following link to troubleshoot cause of problem:
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html

If original problem was caused by sediment burning out lower element, did you clean sediment out of water heater when changing lower element?
Two element water heaters are 240V.
Thermostats cut power on one hot wire, but 240V circuit has 2 hot wires. So lower element has power at all times coming from the red wire.
1) Use ordinary tester or multimeter set to 240V <> test each screw of lower element to bare metal part of water heater. Each screw should show 120V. If there is no power, then upper thermostat is defective, screw on upper thermostat is loose, or red wire between upper and lower parts of water heater is bad.
2) Remove wires from lower element. Use continuity tester or multimeter set to ohms <> test across both screws of element to see if power travels through element <> if element is bad, then continuity tester will not light up and multimeter will show no ohms. Put wires back on element after test.
3) Start with cold water in tank. Turn on water heater & upper element will turn on. Put ear on side of tank to hear if upper element is heating water. Test #3 screw to #2 screw on low left side of thermostat, should read 240V > Test #3 screw to #4 screw on low right side of thermostat, should read 0V.
Put ear on side of tank. When upper element turns off: Test #3 screw to #2 screw on low left side of thermostat, should read 0V > Test #3 screw to #4 screw on low right side of thermostat, should read 240V. If this test shows 0V and 0V, then upper thermostat is bad. If reading is correct, immediately move to next test.
http://waterheatertimer.org/images/Thermostats-numbered-divided-500.jpg
4) Turn temp on lower thermostat to 150. Using ordinary tester or multimeter set to 240V <> Test across both screws of lower element. If there is no reading, and you know element is good from step 2, then lower thermostat is bad, or wire is bad between lower element and lower thermostat, or wire is bad between lower thermostat and top part of heater.
5) Test across both lower thermostat screws, should read 240V.If this test is good, it means wires are good. Put jumper wire across both screws of lower thermostat, and test lower element again. If element finally turns on with jumper wire, then lower thermostat is bad.
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http://waterheatertimer.org/How-to-troubleshoot-electric-water-heater.html

If wire is bad between upper and lower parts of water heater, and water heater has tight-fitting 2" foam insulation, it usually indicates water heater replacement.

Try doing both at the same time - leave it off at least 60 seconds. Also while you have the cover off look fo any buttons (reset). If this does not work you will have to replace the thermostat. (Be sure the power is off to th furnace B4 changing the tstat)