how to activate wifi on my iphone 4s after upgrading to ios 6 ..... wifi button get grayed ???

Try a hard reset and resetting network settings...

Hard reset is by holding BOTH home and sleep/wake buttons down together until you see a white apple symbol... then release both buttons.

See:iOS: Wi-Fi or Bluetooth settings grayed out or dim

It is frequently a hardware problem and an appointment at the Genius bar of an Apple store is in order.

like me and my phone hardware problem >i dont know what can i do they told me mother bord must be change can be re fixed?? 😕