I have a old iPad , but I want to update ios12

Being “fair” has no bearing upon your 8 year-old iPad, which has now been out of support for 3 years, being technically unable to run newer versions of its Operating System. All computer hardware has a finite life; the fact that your technologically very old iPad4 still works at all is testament to the quality of the product.

The 32-bit hardware architecture of iPad4 is obsolete. All modern Apple devices now use 64-bit processors and Operating Systems.

If you need iOS12 (or later), for Apps that have themselves moved-on to requiring newer operating systems, you have no alternative than to replace the iPad with a model capable of running the software.

To update to iOS12 or later, you’ll have no alternative other than than to replace your iPad with a newer model.

If your iPad is currently running, or cannot be updated beyond iOS 10.3.3/4, then have an iPad4 - as suggested by the tags appended to your post.

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Many App Developers have also dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store.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 Once compatible versions Apps are withdrawn, you’ll not be able (re)download or (re)install them.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