i have older version software 9.3.5 on my ipad mini. how to upgrade?

You can't.

iOS 9.3.5 is the highest you can update to with your iPad mini 1.

You need a newer iPad model to update iOS/iPadOS

Your very old 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 iPad mini in September 2016. Your iPad mini cannot be updated to iOS 10 or later major versions of iOS, as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS. This limitation cannot be bypassed - in addition to which, iPad mini is a 32-bit architecture device, whilst all recent versions of iOS/iPadOS are designed for 64-bit hardware platforms.

You may wish to consider that compatible Apps for your iPad are increasingly difficult to find - as third-party developers are increasingly dropping support for older versions of iOS preceding iOS11/12/13. As older versions of Apps are removed from the Apple App Store, you’ll not be able to (re)download or install them - which may severely limit the usefulness and utility of your iPad if you should ever reset or restore your iPad. As such, if you want or need to access most current Apps, requiring more recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model.

Of current models, even the entry-level iPad7 has specifications and capabilities that will massively outperform you current iPad - and is compatible with the vast majority of current Apps.

I hope this information provides helpful and succinct clarity as to your available options - and the limitations of your current iPad mini.

This is never going to happen, ever.

Your iPad is simply too old.

In addition, all 8 and 8-year old iOS devices will be fully & completely obsolete by the end of 2020.

Here’s why.

Tap and read my link below for my additional info about your iPad model.

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

Sorry, but is time for a new, refurbished or much, MUCH “newer” ( read used ) iPad model.

If you decide to purchase another iPad and must purchase older, used iPad models, DO NOT look at anything older than a 2014 iPad Air 2 or 2015 iPad Mini 4.

Both of these models are the oldest iPad models that the up and coming iPadOS 14 will support, in the Fall of 2020.

If you must purchased used devices from unknown sources, you must ask more questions of the iOS device seller, ask for more photo/images, ask if the iPad has user/iCloud activation lock and ask for photos/images/short video proof, ANY REAL POSITIVE VISUAL PROOF, that such an iPad has been reset to factory settings and shown proof of the iOS device's start up screen showing the animated, moving, multi-language “Hello” Welcome to iPad screen.

If the iOS device seller is unwilling to do all of this to show you that the iPad, in question, is ready to use with no barriers to new user activation and use, pass on the iOS device and look at other iOS device sellers elsewhere.

Sorry & Best of Luck to You!