how do I update my ipad to ios 12

Click here for info on using a computer to update: Update your iPhone, iPad, or iPod touch - Apple Support

iPad mini support ends with iOS 9 (9.3.6), while iPad mini 2 support ends with iOS 12 (12.5).

If you’re not sure which iPad you have, see Settings > General > About.

You don't.

Your over 8-year old iPad Mini is too old for any more upgrades/updates.

Sorry.

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 new iPad model OR a much, MUCH “newer” and less expensive, used iPad models, from better known, trusted Apple product/device retailers/resellers, capable of running the latest iPadOS versions.

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

Sorry & Best of Luck to You!