Hi. Do the following to hard reset your Samsung Galaxy Prevail (Turn it Off first)
1. Press and hold both Volume Up + Power button at the same time until you see the Samsung Logo
2. Then release the Power button but continue holding down the Volume Up until you see the Android System Recovery menu
3. Scroll using Volume Down and highligh "Wipe data/factory resee"
4. Confirm with the Power button
5. Scroll using Volume Down and highligh "Yes - delete all user data" Confirm with the Power button
6. Wait until you see "Wiping complete"
7. With "Reboot now" highlighted - select/confirm it using the Power button to exit the recovery mode.