$git push
error: Your local changes to the follwing files would be overwritten by merge:
       a.py
Please commit your changes or stash them before you merge.
Aborting

$git add a.py
$git status
ブランチ master
Your branch and 'origin/master' have diverged,
and have 〇〇 and 〇〇 different commits each, respectively.
  (use "git pull" to merge the remote branch into yours)

コミット予定の変更点:
  (use "git restore --staged <file>..." to unstage)
        modified:   a.py

$ git commit -m "a.pyの変更。など適当に記入しておく"
[master 〇〇] a　ubuntu
 1 file changed, 0 insertions(+), 0 deletions(-)
 mode change 〇〇 => 〇〇　a.py

$ git push
Username for 'https://github.com': 自分の登録名
Password for 'https://〇〇@github.com':　登録してるパスワード
To https://github.com/〇〇.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to 'https://github.com/〇〇.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.

$ git pull
Username for 'https://github.com': 自分の登録名
Password for 'https://〇〇@github.com':　登録してるパスワード