how to upgrade Ipad 1 to iOS 5.1.1

See Updating iOS.

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Update your iPhone, iPad, or iPod touch

Scroll down to the section titled: Update your device using iTunes

If you have an iPad 1, the max iOS is 5.1.1. For newer iPads, the current iOS is 7.1.2. The Settings>General>Software Update only appears if you have iOS 5.0 or higher currently installed.

You can no longer update to iOS 6.x, or down grade the iOS.

iOS 5: Updating your device to iOS 5 or Later

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 the iPad to the computer, open iTunes. Then select the iPad under the Devices heading on the left, click on the Summary tab and then click on Check for Update.

Tip - If connected to your computer, you may need to disable your firewall and antivirus software temporarily. Then download and install the iOS update. Be sure and backup your iPad before the iOS update. After you update an iPad (except iPad 1) to iOS 7.x, the next update can be installed via wifi (i.e., not connected to your computer).

Tip 2 - If you're updating via wifi, place your iPad close to your router to preclude getting a corrupted download.

To upgrade using iTunes:

First, connect your iPad to your PC or Mac using the cable provided when you purchased your device. This will allow iTunes to communicate with your iPad.

You will also need the latest version of iTunes. Don't worry, you will be prompted to download the latest version when you launch iTunes. Once it installs, you may be asked to setup iCloud by logging into your iTunes account. If you have a Mac, you may be prompted on whether or not you want to enable the Find my Mac feature.

Now you are ready to begin the process:

1. If you upgraded iTunes earlier, go ahead and launch it. (For many, it will launch automatically when you plug in your iPad.)
2. Once iTunes is launched, it should automatically detect that a new version of the operating system exists and prompt you to upgrade to it. Choose Cancel. Before updating, you will want to manually sync your iPad to make sure everything is up to date.
3. After canceling the dialog box, iTunes should automatically sync with your iPad.
4. If iTunes doesn't automatically sync, you can manually do it by selecting your iPad within iTunes, clicking on the File menu and choosing Sync iPad from the list.
5. After your iPad has been synced to iTunes, select your iPad within iTunes. You can find it on the left side menu under Devices.
6. From the iPad screen, click on the Update button.
7. After verifying that you want to update your iPad, the process will begin. It takes a few minutes to update the operating system during which time your iPad may reboot a few times.
8. After updating, you may be asked a few questions when your device finally boots back up. This is to account for new settings and features.

 Cheers, Tom 😉