[Git] How to resolve non-fast-forward problem?

Problem
As I try to push my branch to GitLab

git push origin fix_issue6

I get this error message when I do push:

To https://gitlab.com/xxx/xxx.git
 ! [rejected]        fix_issue6 -> fix_issue6 (non-fast-forward)
error: failed to push some refs to 'https://gitlab.com/xxx/xxx.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

How-To
Here has the resolution process: