Make sure you are using the latest version of iTunes on your computer (11.0.1). If you aren't, first update iTunes (iTunes>Check for Updates on a Mac, Help>Check for Updates on a PC). Then connect your phone to your computer, open iTunes, go to the View menu at the top and click Show Sidebar. (If iTunes gives you a pop-up message offering to update your iPad, decline.) Next, right-click on the name of your iPad on the left side of iTunes under Devices and choose Back Up. When the iPad is finished backing up, go to the Summary tab to the right, click "Check for Update" (near the top right), and proceed to update your iPad. iTunes will download the latest iOS for your iPad, go through a number of steps to install it to your iPad, and eventually restart and reappear on the left side of iTunes. You will then be given the option to restore from the iTunes backup you made at the beginning to return the data and settings to your iPad.
Note: if you have the iPad 1, this will update it to iOS 5.1.1. If you have an iPad 2,3 or 4 this will update it to iOS 6.0.1.