The 2013 iPad Air 1, iPad Mini 2 and 2014 iPad Mini 3 cannot upgrade/update beyond any version of iOS 12.x.x.
The internal hardware inside of these iPad models is not powerful enough to run any of the more powerful and advanced features of iPadOS.
Sorry.
If you need an iPad that can run the latest and greatest versions of iOS, as well as, the most current apps available, then, you need to purchase a new iPad from Apple or a discounted, refurbished iPad from Apple’s Refurbished & Clearance section of Apple’s Website or a new or refurbished iPad from an Apple authorised, third party retail or online reseller OR other trusted sources for new or used Apple products.
If you must purchased used devices from unknown, untrusted sources, you must ask A WHOLE LOT 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 POSITIVE, UNQUESTIONABLE, PHYSICAL/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, who will be more accommodating to answering questions and providing visual image or video proof of a proper factory reset iPad.
Sorry & Best of Luck to You!