How do I get iOS 14 on my phone

You cannot. iOS 14 is no longer available for any device. Though even if it was an iPhone 6 Plus cano only update as far as iOs 12.5.5..

If going to Settings->General->Software Update does not offer an update to iOS 12, then you will need to use a computer to update: link-> Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

An iPhone 6 can't be updated beyond 12.5.5, it doesn't have the hardware needed to run a higher iOS.