Share ios14 software update

That's correct. Your phone is too old to run anything later than iOS 12. You will need a new phone if you want to use those apps. Most developers have stopped supporting iOS 12.

Hello,

When updating, your device will update -if it can- to the highest version currently available that it will support. Here are two different ways to ensure the highest iOS that your device will support is installed on your device.

For instructions about updating your iPhone, iPad, or iPod touch wirelessly, click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Only iOS 12.5.7 is currently available for an iPhone 6 Plus

That's not possible, sorry.

See why in this compatibility chart: