I want to update to 16.6

Here are two different ways to ensure the highest iOS that your device will support is on your device. Your device will either update to the highest version it will run that's available, or it won't.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Assuming you own a iPad mini 2, it cannot be updated past 12.5.6