My face bumps the screen when I’m on the phone how do I turn off the screen

The screen should turn off automatically during a call when the light and proximity sensor is covered up by your face or other object.

If it is not doing this there may be something wrong with it.

You can try standard troubleshooting techniques:

1. A Reset may cure some bugs. Hold down the Home and sleep/power buttons until the apple logo shows up.

   If still not resolved:

2. Reste al Settings. Go to Settings->General->Reset->Reset All Settings

   If its still not resolved:

3. Restore the device as new.

   Restore your iPhone, iPad, or iPod to factory settings - Apple Support