How can I download an app from apple store

Read this document to find out how to download apps and games on your iPhone

Download apps and games on your iPhone or iPad - Apple Support

Note that some apps may require you have a newer iOS version than your iOS 12. If Instagram is one of those, you cannot upgrade your phone much beyond its current 12-something to 12.5.6. You will need to buy a newer device.

If you have an iPhone 6 running the latest available iOS version, iOS 12.5.6 your iPhone is too old to run some of the modern day apps.

As the iPhone cannot be upgraded further due to hardware limitations, it can be that you need a new iPhone in order to use apps that need an iOS version above 12.5..6