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To reset the phone, you will need to restore it to its factory settings, then reinstall everything from a backup again.
To reset the phone:
1. Turn off the phone.
2. Press and hold at the same time Volume UP,Home and Power keys.
3. When you see Galaxy logo release the Power key but continue to hold the Volume Up key and the Home key.
4. Release them when the Android System Recovery screen appears.
5. Use Volume Up/Down keys for navigation and Power On key for OK.
6. Choose "wipe cache partition", "Yes -- delete all user data", "reboot system now".
The phone will now be reset and you can set it up as new again.