How to download Instagram

As the message on your screen suggests, you can ONLY download Instagram on an iPhone which can run iOS 15.1 or higher. You own a 10 year old iPhone, which cannot be updated past iOS 12.5.8. So the answer is simple. If you to run Instagram on an iPhone, you need to retire your now ancient iPhone and buy a new or newer, more modern iPhone.

Apple has nothing to do with this. Instagram is owned by Meta, the parent company of Facebook as well and they have chosen to no longer support their app on old out of date devices.