updating iPad from 10.3.3 to 12 iOS

The simple answer you don't, because your 8 year old iPad is too old, too under powered, no longer supported and completely incapable of running the now required 64 bit software needed for the newer iOS levels.

I'm sure you've run into many apps no longer running on your iOS level and apps you may want to download requiring newer iOS levels. This means it's time to retire that 8 year old iPad and buy a more modern newer iPad capable of running the newest levels.

Sorry. Not what you wanted to hear, but is the reality.

Sorry, not possible...

If your iPad is currently running, or cannot be updated beyond iOS 10.3.3/4, then have an iPad4.

Update support for iPad4 ended September 2017. The final iOS releases for this model were iOS 10.3.3 (WiFi Only models) and iOS 10.3.4 (WiFi & Cellular). iPad4 cannot be updated to any later major version of iOS as it lacks the minimum hardware requirements needed to run more recent versions. The 32-bit platform upon which the iPad4 is based is effectively obsolete.

Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store. Withdrawal of Apps supporting older models of iPad is making their utility and usefulness significantly more limited. Once compatible versions Apps are withdrawn, you’ll not be able (re)download or (re)install them.

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 supports current iPadOS 13.x - and has hardware specifications that will vastly outperform your old iPad4.

I hope this information is helpful - if only in clearly stating the limitations of your iPad4 and your available options.

A 4th generation iPad cannot update beyond iOS 10.3.3 (10.3.4 for cellular models). It is the last iOS version it can run. Its 8 year old hardware at this point.