After updating my iphone 6, it doesn't seem to be able to fetch the wifi signal properly. What should I do?

If you have not already powered off the phone, waited a minute, then powered the phone back up again.....now would be a good time to do this.

Also try turning Airplane Mode on for a few seconds, then turn it back off again.

Would not hurt to power off your Wi-Fi router, wait a minute, then power it back up again.