下載 iOS 5.1

Homan

Apple 隨 iPad 3rd Generation 一起發佈了 iOS 5.1
iOS 5.1 比 5.0 除了 bugs fixed 外，亦加了一系列的新 features:

...... including the ability to delete images from Photo Stream, camera shortcut visible by default on lock screen, improved camera face detection, Japanese language support for Siri, genius mixes for iTunes Match, updated AT&T network indicator, improved battery life, and more.

from OSXdaily

不過，小弟發現我用 iTune 也好，用 iPhone 的 OTA update (即不插電腦直接在 iPhone 中 update) 都 hang 住 download 不了。
後來發現一個小技巧，可以順利 download iOS 5.1

如有同樣問題，可以試試方法如下：
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在 Network preference 中的 Advance 內，挑選 DNS，再 add 一個 8.8.8.8 的 DNS


OK 後 reset network，問題就解決了！

Homan

如果出現「your device isn't eligible for the requested build」而不能 update（我正正遇到這問題）
可以嘗試以下方法：

To fix this, open your hosts file and remove (or comment) any line that has something like as shown below:

    74.208.151.171 gs.apple.com

In order to access your hosts file, see the instructions below.

Apple OS X Lion

    Open Terminal
    Copy and paste the following code into Terminal
    sudo nano /private/etc/hosts
    Press Enter.
    Remove 74.208.151.171 gs.apple.com or add ‘#’ in front of this line (commenting).
    Save by pressing Command+S, click Save.

Apple OS X Snow Leopard & Leopard

    Open Terminal (Click Spotlight, or press Command+Space. Now type terminal.)
    Copy and paste the following code into Terminal
    sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit /etc/hosts
    Press Enter.
    Remove 74.208.151.171 gs.apple.com or add ‘#’ in front of this line (commenting).
    Save by pressing Command+S, click Save.