How I updated ipad 4 to iOS 12

Sorry, but update of your iPad to iOS 12 isn’t 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.

Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store.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 Once compatible versions Apps are withdrawn, you’ll not be able (re)download or (re)install them.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 both the limitations of your current iPad and your available options.

You can't is the simple answer. iPad 4 is too old, too under powered to run the required 64 bit software, is no longer supported and cannot be updated beyond iOS 10.3.5. Sorry.

If you need the newer iOS versions for most modern apps, it's time for a newer (iPad 4 is now over 8 years old) more modern iPad.

You don't.

Tap and read my link below for my additional info about your 8-years old iPad model.

If you own OLD 8 and 9 Year Old iPad mo… - Apple Community

Sorry & Best of Luck to You!