“But how to install applications from App Store ? I recently purchased iPhone 6 Plus”
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Installing Applications on the App Store:
If it’s available to you, you’d have to tap “Get” and see what pops up from there. If it’s dulled out, then it’s just not an option.
Getting it From the Developer:
If you can find out who the Developer is in the app’s site of the App Store, and then ask the Developer if they have anything on their end that works(perhaps an alternative, if not exactly what you are looking for).
Don’t Purchase Apps from a Third-Party:
When getting an app, don’t purchase if from any olde Web site. I recommend purchasing it from none other than Apple or the Developer, as a way to avoid scams and such.
In all…Take this as a good reason to upgrade to a much more recent iPhone model. Apple typically provides support for their hardware and software for 7 years, and then ends it, being technology has changed so much. Your device was released way pre-COVID, and since that outbreak, technology has taken a major rise to meet the needs of consumers(ie hardware and software running faster because of working out-of-the-house.