iOS 9 brings its own share of inherent bugs and post-update issues such as persistent Wi-Fi connectivity problems like any other major iOS update. Some of the recently reported issues include inability to connect to a Wi-Fi network, unexpected error messages with incorrect password entries, intermittent connection issues and/or extremely slow connection speeds.
Reset network settings
Several connectivity problems can easily be resolved by simply resetting the network settings, which will clear the memory caches and DHCP settings to get the default or factory recommended settings. Here is how to reset network settings: Open Settings app > go to Settings > General > Reset and tap on Reset Network Settings.
Force Restart
Force restarting your iOS device is the preliminary step that you should try before anything else. Press and hold Sleep/Wake and Home buttons simultaneously for at least ten seconds and then release them once the screen shows the Apple logo, following the reboot.
Disable Wi-Fi networking services
Several users have reportedly resolved Wi-Fi connectivity issues by simply disabling Wi-Fi networking services. Here is what you should do: Go to Settings > Privacy > Location Services > System Services and disable Wi-Fi networking services from there. Note: This will only disable location services for Wi-Fi networking, but it will not affect Wi-Fi functionality entirely.
Forget Wi-Fi network to fix password incorrect prompt
Those who are unable to connect to any Wi-Fi network due to unexpected errors with password prompts (despite entering the right password), can try forgetting the Wi-Fi network and then attempt rejoining the network.
To accomplish this, go to Settings > Wi-Fi and then select your Wi-Fi network from the list by tapping on it. Then tap on Forget This Network option. A pop-up message will appear on screen asking your confirmation to forget the chosen Wi-Fi network. Just tap on Forget option to forget the network.
Now try rejoining the Wi-Fi network by going to Settings > Wi-Fi and then selecting the same network again. Enter the password when prompted and you are all set.