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If it supports a higher iOS version : Update your iPhone, iPad, or iPod touch - Apple Support

Hi Clhloe,

From what I know, u tap on Settings->General->Software Updates, if there’s any updates for your iPad, they will appear over there, just tap on install updates but pls make sure your battery level is above 50% and pls switch off any VPN network connection before software updating in order for it to work smoothly. Hope this helps.

Software update in iTunes using a computer is an option too. (What MichaelPM said is true too)

Clhloe, if you want to play safe, it’s better that you bring your iPad to Apple Store for Apple Technicians to do the updating for you if your iPad is still under warranty.

Nevermind.

Your signature indicates you own a 9.7 inch screen iPad Pro.

You should be able to upgrade to iOS 12.4 via iTunes on computer.

The OP is too far behind in iOS updates.

The OP will need to connect their iPad to a computer running iTunes to do the iOS software update via the Software Update feature in iTunes.