Samsung Galaxy SII ( GT-i9100) - Factory Reset or Hard Reset

Note: Performing these steps you will loose all your contacts, messages, all application settings and all installed applications.
If you have trouble with phone software and you want to reset the phone to factory state, you can try factory reset using below steps:
1. Go to System Settings and then select Privacy.
2. Select FACTORY DATA RESET
4. Select RESET PHONE
5. If the password is required,enter it. Then confirm by selecting ERASE EVERYTHING. 

If you’re unable to access the settings menu of your Galaxy S II, to Hard Reset the try this method:

1. Turn phone POWER OFF, or pull the battery out and put it back again.
2. Press the Volume UP + Home Button.
3. Keep pressing these 2 buttons + press POWER button for 2-3 seconds.
4. Release only power button but keep pressing Volume Up, Home button.
5. A Boot Menu will then show, Use the volume up/down to choose and home to select.
6. Select Wipe data/factory reset option and press Power button.
7. Now select Yes - delete all user data and Press Power button.
8. Once Wipe Completes, select reboot the system now.

This reset method is especially useful if your phone continually freezes or fails to respond or you’ve forgotten the security locks you’ve set up to access the phone.