Fix Wifi Disconnecting Issue After iOS 6.1.2 Update

Method 1:

• Open 'Settings' then go to 'General'.
• Scroll down and tap on Reset.
• Now tap on "Reset Network Settings"

Method 2:

• If the first method doesn't works, then you might update to iOS 6.1.2 using OTA.
• You can now download the ipsw file from here for your device and restore your iPhone using iTunes.

If both methods doesn't work then you bring your iPhone to the nearest Customer Care Center.