how can we update iPad mini 2 software 9.3.5 to 12.5.3

In making explicit reference to iOS 9.3.5 it is evident that you have misidentified your iPad mini. With iOS 9.3.5 installed you must have a “first generation” iPad mini - and not an iPad mini2. This misidentification is not uncommon.

Not sure which iPad you have? Identify your iPad model.

A first generation iPad mini can only be updated to iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular models).

Apple ended update support for first generation iPad mini in September 2016. An iPad mini1 cannot be updated to iOS 10 or later major versions of iOS, as the internal hardware does not meet the minimum technical requirements for newer versions of iOS/iPadOS. This limitation cannot be bypassed - the iPad mini1 is a 32-bit architecture device, whilst all recent versions of iOS/iPadOS are designed for 64-bit hardware platforms. 32-bit devices are now obsolete.

iPad mini2 and mini3 are similarly limited to iOS 12.5.4. For these models of iPad, Apple ended major update support in September 2019. Since then, there have only been “point” security updates. These models of iPad mini are are 64-bit architecture devices, but have insufficient RAM for further iOS update.

In having an older model iPad mini, you need to understand that compatible Apps for older iPad models are becoming difficult to find; third-party developers are increasingly dropping support for 32-bit devices and iOS preceding iOS11/12/13. 

As such, if you want or need to access most current Apps, requiring more recent versions of iOS than your iPad can support, you’ll need to consider replacement of your iPad with a much newer or current model.

I hope this information and clarification is helpful in explaining your inability to update the iPad mini.

Upgrading/updating that iPad any farther may never, ever be possible.

If your iPad model has been stuck on iOS 9.3.5 for all of these past years, then it is possible you own an almost 9-year old, 2012 iPad Mini 1st generation and NOT a 2013 iPad Mini 2nd generation.

That iPad model cannot ever upgrade beyond iOS 9.3.5/9.3.6.

Your nearly 9-year old, iPad Mini model is too old, now.

Sorry.

Apple ended support for all 8-10-year old iOS devices nearly 5 years, ago, in 2016.

Third party app developers abandoned and stopped supporting these 8-10-year old iOS devices nearly 3 years, ago, in 2018.

There are, virtually, NO MORE third party apps available from the iOS App Store, any longer!

Third party compatible apps for such old devices are few and far between, now, in 2021.

This includes apps for both iOS 9 AND iOS 10 devices!

Simply put, a 1st gen iPad Mini is too old and obsolete now.

Sorry, but this is the reality for ALL 8-10-year old AND OLDER iOS devices now. Especially now in 2021!

If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted Apple product/device retailers/resellers, some locally or from online, on the Web/Internet, capable of running the latest iPadOS versions.

There are PLENTY of less expensive, newer, older iPad models that can run the latest, current versions of iPadOS.

If you need to purchase older, less expensive iPad models, look at older IPads from 2014 and later.

I am sorry that you had to learn all of this, but this does not change any of the facts regarding these very old iOS devices.

Sorry & Best of Luck to You!

Update your iPhone, iPad, or iPod touch - Apple Support

if it says it's already up to date then you likely have a ipad mini1 rather than a 2