You can hard reset a phone.
You need to know the make and model number.
From there, you can hard reset the phone that should contain an old back up of the android system. The original version of android that was factory installed on the device.
The process varies from phone to phone or android version used.
And is performed by holding in the power key for ten seconds or more or a combination of keys on the phone such as the volume up and down keys together just after powering the phone or device on.
Just do a google seach for your make of phone and its model number with the words factory restore or hard reset.