Can you upgrade storage on an iPhone 6? If so how and how much does it cost?

I have an iPhone 6 with 16GB storage but it's not enough for what I need. I used to have 32GB before but this is not an option on the iPhone 6. How can I upgrade to the next amount up (64GB) without changing my phone? And how much would it cost?

NO, nothing. Sadly iPhones are not designed to have their storage upgraded.