You cannot update iPad 2 to iOS 5.1. Your only option is to upgrade to the latest version of iOS 6....and you will lose Google Maps and YouTube. Some apps will allow you to play them, but I have read that the current Google Map app is not integrated into your iPad as is the originally installed version.
From the iOS version shown in your signature you must update the iOS on your iPad by connecting it to a computer.
Run iTunes on the computer you normally sync with, connect your iPad, select your iPad in iTunes and select general in the row of tabs that are presented. scroll down under General to find Check for Updates.
If you have an iPad 1, the max iOS is 5.1.1. For newer iPads, the current iOS is 6.0.1. For the iPad Mini the iOS is 6.0.2. The Settings>General>Software Update only appears if you have iOS 5.0 or higher currently installed.
iOS 5: Updating your device to iOS 5 or Later
How to install iOS 6
iOS: How to update your iPhone, iPad, or iPod touch
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 anitvirus 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 6.x, the next update can be installed via wifi (i.e., not connected to your computer).