My iPad says it is up to date with iOS but the software is 12.4.7 not 13

Amplifying comments already offered by Neil, your iPad mini3 can only be updated to iOS 12.4.7.

Apple ended update support for iPad mini3 in September 2019 - and other than a few “point” security updates, no additional feature updates have since been offered. iPad mini3 cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with your iPad mini3, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS13.x.

Sorry, I appreciate that this may not be what you were hoping to learn.

I hope this information provides helpful clarity as to your available options - and the limitations of your current iPad mini3.

You can't. An unit running 12.4.7 doesn't have enough RAM for 13 or above.

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You’re very welcome. I’m delighted to have been able to help.