How can i install the 15.2 IOS upgrade?. I have installed 11.4 IOS version on my iPad.

iPad model number A1673 corresponds to a 2016, 9.7 inch screen iPad Pro.

That iPad can be upgraded to the latest iPadOS 15.2

However, your iPad is waaaay behind in iOS/iPadOS upgrades/updates.

If you have access to a full computer, like a Mac or Windows PC, try either using the Finder on a Mac that has the newest macOS Catalina, or later macOS, installed OR with an older Mac with an older Mac OS, download and install the latest iTunes version ( or latest iTunes for Windows PC ), make sure the Mac or Windows PC is connected to the Internet, connect your iPad by the USB charge cable to the computer and let the iTunes application upgrade/update that iPad model, using the iTunes Software Update feature, to its current, maximum OS of iOS 12.5.5/iPadOS 15.2.

Make sure your iPad is, at least, charged to 50% or greater to attempt this.

Also, make sure to do a backup of your iPad's data, prior to upgrading/updating.

Backup to Apple iCloud and/or a full Mac or Windows PC using the same iTunes app OR, if a newer Mac running macOS Catalina, or later, making a backup using the macOS Finder app.

If NO access to a Mac or Windows computer, you will have to seek help from a good friend or family member, with a full Mac or Windows PC, to help you out

OR

Search online to locate and call contact your nearest, “local” Apple Store location to try/attempt to make an appointment to get your Apple iPad upgraded/updated to its latest iPadOS 15 version.

Apple Store employees will be glad to help/assist you.

Another issue.

You own a, relatively, low storage iPad model.

Apple just put an end to 32 GBs devices, in September 2021, with the base model iPad now offering 64 GBs of internal data storage ( approx. 50 GBs of actual, usable internal data storage ), which, for a few more years, anyways, has enough storage to make an iPad useful for longer period of time before data storage fills up.

Purchasing a low storage Apple device, to save on the cost of the device, is a false economy.

A 32 GBs storage iPad starts out with approx 30 GBs.,after initial internal flash drive formatting.

Minus approx. 6.5 GBs for iOS/iPadOS leaves between 23-24 GBs.

Minus 3-6GBs, for iOS/iPadOS system operational reserve, that MUST BE MAINTAINED AT ALL TIMES, to ensure/insure normal, expected operation of iOS/iPad OS, leaves approx. between 18-20 GBs of actual, usable data storage space.

Even with 20 GBs of actual, usable internal data storage space in an Apple iOS/iPadOS device,

a 32 GBs data storage devices are, now, more quickly, becoming the “new” low storage 16 GBs internal data storage space devices, today, a 32 GBs Apple iOS/iPadOS device has 3-4 times the internal data storage space of older base model, low internal data storage,16 GBs. internal data storage Apple iOS/iPadOS devices.

In reality and in use, in 2021, a 32 GBs internal data storage device is kind of a joke and of limited usefulness, when up to 2TB of internal data storage ( approx. 1.8 TBs or usable internal data storage ) portable tablet devices exist today.

Best of Luck to You!

A little hard to help/advise you when you provide little to NO useful information about your iPad.

Here are my questions to you.

Can you, please, confirm what year and model iPad do you have/own?

The “A” model number can be found on the aluminium back of your iPad, in very small print, right under the larger bold print “iPad” on the back of the iPad.

What iOS/iPadOS version is this iPad, currently, running?

What is the rated, internal data storage space/capacity of your iPad model and how much of this free, internal data storage capacity is left inside of your iPad model?

All of that additional info can be found in the Settings app, General settings panel, About.

Please try to make an attempt to answer all questions asked.

Thank You.