how to update ios on ipad 1

Connect your iPad to iTunes, and under the iPad tab that appears press "Check for updates". You'll then either be asked whether you'd like to update your software, or be told that your software is up to date.

It's possible that your iPad has the highest version of software it is capable of running, and the apps you are trying to download require a newer version of the software. If that's the case, there's nothing you can do I'm afraid.

I have that problem too. I have iPad first generation and has it's latest version of its. Everytime I download apps there's a warning telling me to update the latest version. I tried to do it, but when checking the update to general, it says i have it's latest version which is 5.1.1

The Original iPad can only be updated to IOS 5.1.1

The original iPad 1 cannot be upgraded past iOS 5.1.1. Sorry.