Memory card problems
Is it possible to increase an iPhone memory from 16gb
iPhone 6 Plus
The physical storage on your iPhone 6 Plus 16 gb model cannot be changed.
That being said, you may want to consider an iCloud storage plan and do things like turn on iCloud Messages and iCloud Photos. By turning on iCloud photos, if a lot of your space is used for pictures, this will free up space on your iPhone for things like apps.
iCloud storage plans and pricing - Apple Support
You may also want to remove any apps that you don't need (including some of the built-in ones):
Delete built-in Apple apps on your iOS 12 or later device or Apple Watch - Apple Support
How to delete apps on your iPhone, iPad, and iPod touch - Apple Support