Why is there a version conflict when installing a package through Composer?

Question

Composer

A package with a dependency is installed:
"guzzlehttp/guzzle": "^6.2"
I install a package with a dependency:
"guzzlehttp/guzzle": "^6.3"
I get an error:

Problem 1
    - Can only install one of: guzzlehttp/guzzle[6.3.0, 6.2.2].
    - Can only install one of: guzzlehttp/guzzle[6.3.0, 6.2.2].
    - Can only install one of: guzzlehttp/guzzle[6.3.0, 6.2.2].
    - appwilio/cdek-sdk dev-master requires guzzlehttp/guzzle ^6.3 -> satisfiable by guzzlehttp/guzzle[6.3.0].
    - Installation request for appwilio/cdek-sdk dev-master -> satisfiable by appwilio/cdek-sdk[dev-master].
    - Installation request for guzzlehttp/guzzle (locked at 6.2.2) -> satisfiable by guzzlehttp/guzzle[6.2.2].

The caret ^ means that any version greater than or equal to the current version up to version 7 is suitable - in theory, composer should update guzzle to version 6.3, but this does not happen.
Why?
Composer version latest - 1.6.3
Resolved : The problem was solved by removing the already installed package with the requirement ```^6.2```, manually installing guzzle version 6.3, installing the removed package and installing the package that was originally needed.