a lot of instruction to Galaxy S8 can be found here hard reset of Samsung Galaxy S8

Reset Network Settings:

NOTE: Resetting network settings will delete connection data including Wi-Fi and Bluetooth networks. Background data sync, data restrictive settings, and network selection mode will be switched to “on”, “default” and “automatic”

1. Open the apps drawer
2. Go to “General Management”
3. Tap “Reset” > “Reset network settings”
4. Tap “Reset settings”
5. Enter your PIN if necessary
6. Tap “Reset settings” to confirm

Soft Reset With Hardware Keys:

NOTE: A soft reset is designed to simulate battery removal. This function can be useful if your phone is frozen and unresponsive. However, this method will not delete any content on your phone. To soft reset the Samsung Galaxy S8:

1. Press and hold “Power” and “Volume down” for up to 45 seconds
2. Wait for the phone to reboot

Hard Reset

NOTE: Hard resetting your smartphone will delete all user data stored on the phone’s internal memory, including downloaded content, contacts, media files, ringtones, applications etc. Additionally, all settings will be restored to factory values. However, the SIM and SD cards will not be affected by the hard reset.

Hard Reset via Settings:

1. Open the apps drawer on the home screen
2. Go to “Settings” > “Cloud and accounts”
3. Tap “Backup and restore”
4. Optionally, enable or disable “Back up my data” and “Restore”
5. Tap “Back” to return to “Settings” and go to “General Management” > “Reset” > “Factory data reset”
6. Tap “Reset device” and if necessary, enter your PIN / password to proceed
7. Tap “Continue” > “Delete all” to confirm

Hard Reset Using Hardware Buttons:

NOTE: if your phone freezes, crashes, or otherwise prevents you from hard resetting it via Settings as described above, you can try resetting the phone using hardware buttons.

1. First turn off the smartphone
2. Then, press and hold “Volume Up” and “Home”, then press and hold “Power”
3. At the “Android” logo, release all buttons and wait for 30-60 seconds for the Android system recovery menu
4. In the Recovery menu, press “volume down” repeatedly to navigate to and highlight “Wipe data / factory reset”
5. Press “Power” to select the highlighted option
6. Press “Volume down” repeatedly to highlight “yes – delete all user data”
7. Press “Power” again to confirm and reset your phone
8. Once the process is complete, highlight “Reboot system now” and press “Power” one last time to reboot your device