Auto mode relies upon a programmed time and temperature setting to know when to turn on your HVAC.
First, replace the batteries to ensure the thermostat will save your settings.
Next, reset the AC temp you would like in your home across each day segment ... there are typically four program periods per day (wake, depart, return, sleep).
Next, reset the Heat temp you would like in your home across each day segment.
Then, set your thermostat to Auto and the program will do the rest.
NOTE:
Most thermostats have a Hold setting that allows you to skip the programmed settings and hold a constant temp across all day segments.
You can release the hold setting by pressing Run Program to return to the program schedule.