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Download and install iOS 5  (How To Guide) – IPSW.You can now install iOS 5 on your iOS device using this direct download links.

a) Install iTunes 10.5 beta from direct link here.(Mac – Windows 32 bit – Windows 64 bit)
b) Now you need to download the right firmware file for your iOS device:

• iPhone 3GS
• iPhone 4 (GSM, CDMA)
• iPod 3G
• iPod 4G
• iPad 1G
• iPad 2 (Wi-Fi, GSM, CDMA)

 c) Now open iTunes that you downloaded above and restore using your firmware.

Update: Here’s a guide to install iOS 5 without developer account.