Q: when upgrade my iphone 4S to iOS 9.3 front camera not work

Update could not cause hardware issue. Take your device to the Apple Store for further diagnostics.

You may also check this article: Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support

This is hardware issue. Most likely it has no relation to new  update. Updates do not cause hardware problems. Check the article above.

You may also try basic troubleshooting techniques:

Try to reset your device. This won't delete your data stored on this device.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

If this does not help, restore your device to factory settings. Note that this will erase the data from your device.

Take a look this Apple Support article: Use iTunes to restore your iPhone, iPad, or iPod to factory settings - Apple Support.

Try Installing iOS via DFU Mode via iTunes in PC

If this isn't Fix bette to Take the Phone to a Repair Store !!