how do i update an old ipad mini from 9.4.3 tp ios 12

If your iPad Mini is a First Generation iPad Mini, it cannot be updated beyond iOS 9.3.5

Identify your iPad model - Apple Support

If its a Second, Third or Fourth Generation iPad Mini follow the steps here:

Update your iPhone, iPad, or iPod touch - Apple Support

Here's a list of every Apple device that supports iOS 12:

• iPad Air 1, iPad Air 2, iPad Pro (12.9, 2015), iPad Pro (9.7), iPad 2017, iPad Pro (10.5), iPad Pro (12.9, 2017), iPad 2018
• iPad mini 2, iPad mini 3, iPad mini 4
• iPhone 5s, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone SE, iPhone 7, iPhone 7 Plus, iPhone 8, iPhone 8 Plus, iPhone X, iPhone XR, iPhone XS, iPhone XS Max (iOS 12 is preinstalled on last three)
• iPod touch (sixth generation)

There isn’t an iOS 9.4.3.

If you’re using an iPad 2, iPad 3, or first generation iPad mini on 9.3.5, you can’t.

If you’re using an iPad 4 on 10.3.3, you can’t.

Otherwise, use the Software Update function or update it from iTunes on a computer.

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You can't. Your iPad 2 is not capable of running anything newer then that.

If you need to run iOS 12 you must replace your obsolete iPad with a new one.