How to update ios 5.1.1

Sorry, not possible. Apple ended update support for your original (first generation) iPad in September 2012; it is obsolete.

The final iOS release for iPad1 was iOS 5.1.1. It cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions.

App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn the majority of older versions of Apps from the Apple App Store.

If Apps that you need to use now have higher minimum iOS-version requirements, your only option is to replace your iPad with newer model. Even the basic current iPad7 model supports current iPadOS 13.x and soon to be released iPadOS14 - and has hardware specifications that capabilities will vastly outperform your original iPad. As such, usefulness and utility of older models are becoming increasingly limited - and in some cases, of little use at all.

Sorry, this is clearly not the news that you were hoping to hear.

I hope this information is helpful - if only in clearly stating your current options and potential ways forward.