Unable to download from App Store, iPhone needs update

An iPhone 6 plus can't be updated beyond 12.5.7, it doesn't have the hardware needed to run anything higher.

Most apps these days require a higher operating system than what the iPhone 6 plus can run.

If you need a higher iOS version, you will have to purchase a newer phone.

To update to 12.5.7 -

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support https://support.apple.com/guide/iphone/update-ios-iph3e504502/ios

Sorry, most apps require later iOS versions, but your iPhone model can't be updated past 12.5.7.

See why in this compatibility list:

An iPhone 6 Plus cannot update past iOS 12. Most apps now will not work on a phone that old. You need a new phone that can use iOS 16.4.1.