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When trying to update from iOS 5.1 I keep getting an error message reading "unable to check for update, an error occurred while checking for a software update". The update screen loads for a couple of minutes before showing this.

How can I upgrade to iOS 5.1?

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Are you using the Settings app, or iTunes? –  timothymh Mar 10 '12 at 2:33
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It turns out you might need to set your DNS server to 8.8.8.8 as I found out Here.

To change DNS settings, go to the Settings app and then pick Wi Fi in the left column. Tap the blue arrow that's on the right of your Wi Fi network name, and DNS should be an option, and you'll see a number like 198.0.0.1. Tap this number and change it to be 8.8.8.8.

Try the update after changing the DNS setting (wait a minute as you'll have to reconnect your wireless connection) and the update should work now.

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That is doubtful. It's more likely that changing your DNS servers just refreshed your DNS cache. Something, which can be done by running the following command in Terminal: dscacheutil -flushcache It may be your ISP has some aggressive caching, but I doubt you'd have to migrate over to Google's servers (which incidentally are 8.8.8.8 and 8.8.4.4). –  cksum Mar 10 '12 at 9:54
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