Difference between update definition and refactoring

What is the main difference between update definition and refactoring?
When I use the update definition it works fine as far as I don't have optional elements (it's important that I always check the option "keep existing values"

I used refactoring also, but when after successful refactoring I've done update definition option, it turned out that there are a lot of functions to update.

As a result I don't know when to use those two functions and in which order.

Thanks in advance