Problem:
We want to use your fixed version before project owner accept your pull request.
Solution:
Open your

composer.json

file and in "repositories" array add:

  "repositories" : [ {
    "type" : "git",
    "url" : "https://github.com/yuksbg/guzzle"
  } ]

after that go to "require" section and use your branch/version like this:

  "require" : {
    "guzzlehttp/guzzle" : "dev-bugfix"
  },

Remember: Do not change original user in require section.
After that just

 composer update

and tada :) You will use your fork.