In order to hard reset a Acer Liquid S2, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Acer Liquid S2 device will be brought back to the default configuration of a brand new device.
1. Power the phone off.
2. Press and hold together Volume Down + Power button until the phone vibrates.
3. Now!, quickly release Volume down and press Volume up. You will see a new menu on screen.
4. Next, use Volume buttons to select "Recovery" option.
5. Recovery mode menu will appear. From it, choose "Wipe data/factory reset" using Volume buttons to scroll to options and Menu button to confirm.