Click on "Settings" → "Backup & Reset" → "Factory data reset" to clear all user setting and recover the system to the default status.
Please kindly be noted that some models (like D2-712/D2-912) may require extra step as to select roboot system now. (Note: All installed applications and user personal data will be erased (emails, contacts...etc.). Data saved in the internal storage like photos, music files and others will not be erased.) Make sure to back up all your files first.