Q: I need the IOS 5 for my Iphone 3G

Not possible. iOS 4.2.1 is the highest possible for the iPhone 3G.

Not possible for your model:

See the chart below to determine whether you can upgrade your device and what you can upgrade to.

IPhone, iPod Touch, and iPad iOS Compatibility Chart

     Device                                       iOS Verson

iPhone 1                                      iOS 3.1.3

iPhone 3G                                   iOS 4.2.1

iPhone 3GS                                 iOS 6.1

iPhone 4                                      iOS 6.1

iPhone 4S                                    iOS 6.1

iPhone 5                                      iOS 6.1

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iPod Touch 1                               iOS 3.1.3

iPod Touch 2                               iOS 4.2.1

iPod Touch 3                               iOS 5.1.1

iPod Touch 4                               iOS 6.1

iPod Touch 5                               iOS 6.1

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iPad 1                                          iOS 5.1.1

iPad 2                                          iOS 6.1

iPad 3                                          iOS 6.1

iPad 4                                          iOS 6.1

iPad Mini                                     iOS 6.1

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Select the method most appropriate for your situation.

Upgrading iOS

   1. How to update your iPhone, iPad, or iPod Touch

   2. iPhone Support

   3. iPod Touch Support

   4. iPad Support

     a. Updating Your iOS to Version 6.0.x from iOS 5

          Tap Settings > General > Software Update

     If an update is available there will be an active Update button. If you are current,

     then you will see a gray screen with a message saying your are up to date.

     b. If you are still using iOS 4 — Updating your device to iOS 5 or later.

     c. Resolving update problems

        1. iOS - Unable to update or restore

        2. iOS- Resolving update and restore alert messages

Only way you can get iOS 5 is new iPhone.

Allan