You can’t install an app on your iPhone, iPad, or iPod Touch unless you buy a new phone, a new iPad, and/or a new iPod Touch.
You can install an internet-enabled chat software that works on your device, but you need a new smartphone.
Here’s how to get it on the Apple App Store.1.
Find a phone you want to install an Internet chat software on.2.
Pick an app that works with that phone.3.
Install the software.4.
Enjoy the chat software.
For more tips, see the iPhone 6S guide.