See
iOS 5: Updating your device to iOS 5
http://support.apple.com/kb/HT4972
iOS: How to update your iPhone, iPad, or iPod touch
http://support.apple.com/kb/HT4623
If you are currently running an iOS lower than 5.0, connect to your computer & click on the "Update your device using iTunes".
Tip - You may need to disable your firewall and anitvirus software temporarily. Then download and install the iOS update. After you update to iOS 5.x, the next update can be installed via wifi (i.e., not connected to your computer).
Look at iOS Troubleshooting Wi-Fi networks and connections http://support.apple.com/kb/TS1398
iPad: Issues connecting to Wi-Fi networks http://support.apple.com/kb/ts3304
iOS: Recommended settings for Wi-Fi routers and access points http://support.apple.com/kb/HT4199
Additional things to try.
Try this first. Turn Off your iPad. Then turn Off (disconnect power cord) the wireless router & then back On. Now boot your iPad. Hopefully it will see the WiFi.
Go to Settings>Wi-Fi and turn Off. Then while at Settings>Wi-Fi, turn back On and chose a Network.
Change the channel on your wireless router (Auto is best). Instructions at http://macintoshhowto.com/advanced/how-to-get-a-good-range-on-your-wireless-netw ork.html
Another thing to try - Go into your router security settings and change from WEP to WPA with AES.
How to Quickly Fix iPad 3 Wi-Fi Reception Problems
http://osxdaily.com/2012/03/21/fix-new-ipad-3-wi-fi-reception-problems/
If none of the above suggestions work, look at this link.
iPad Wi-Fi Problems: Comprehensive List of Fixes
http://appletoolbox.com/2010/04/ipad-wi-fi-problems-comprehensive-list-of-fixes/
Fix iPad Wifi Connection and Signal Issues http://www.youtube.com/watch?v=uwWtIG5jUxE
Fix Slow WiFi Issuehttps://discussions.apple.com/thread/2398063?start=60&tstart=0
Unable to Connect After iOS Update - saw this solution on another post.
https://discussions.apple.com/thread/4010130
Note - When troubleshooting wifi connection problems, don't hold your iPad by hand. There have been a few reports that holding the iPad by hand, seems to attenuate the wifi signal.
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If any of the above solutions work, please post back what solved your problem. It will help others with the same problem.
Cheers, Tom 😉