Can I update my iOS without erasing?
Can I update my iOS 5.1.1 without erasing everything from my iPad?
iPad 2, iOS 5.1.1
No. The updater must first erase all memory before installing the new firmware. Be sure you sync your iPad with iTunes to assure you have a current backup on the computer that you can later restore after the firmware is updated.
iOS- How to update your iPhone, iPad, or iPod touch
Please get the iPad iOS 4 user guide.
Please Get the iPad User Manual for iOS 5