How do I enable auto change over on the TH6110D1005, TH6220DXXXXX, TH6320U1000?

1. To begin, press and hold the Up arrow and FAN buttons until the display changes.
2. Press Up or Down to change settings.
3. Press NEXT to advance to next function.
4. Press DONE to exit and save settings.

ISU 12 needs to be set to 1 for Heat/Cool/Auto/Off to be available

If set to 2 the only system option will be Auto

If your thermostat does not have option 12 it is an older generation where this option was not available