How to fix ! [rejected] main -> main (non-fast-forward) error: failed to push some refs in Git

First, download the remote changes without modifying your working directory:

git fetch origin

This updates your remote-tracking branches (like origin/main) so you can see what's different.

Before merging, understand what commits exist on the remote that you don't have:

git log HEAD..origin/main --oneline

This shows commits on origin/main that aren't in your local branch. You can also see a visual comparison:

git log --oneline --graph --all

The simplest approach is to merge the remote changes into your local branch:

git pull origin main

Or if you've already fetched:

git merge origin/main

This creates a merge commit that combines both histories. If there are conflicts, Git will pause and let you resolve them.

For a cleaner, linear history, rebase your commits on top of the remote changes:

git pull --rebase origin main

Or if you've already fetched:

git rebase origin/main

This replays your local commits on top of the remote branch. If there are conflicts, resolve them and run:

git rebase --continue

After integrating the remote changes, push again:

git push origin main

The push should succeed because your local branch now includes all commits from the remote.

If you used git commit --amend or git rebase to intentionally modify commits that were already pushed, you may need to force push:

git push --force-with-lease origin main

WARNING: This overwrites the remote branch. Only do this if:
- You're the only one working on this branch, OR
- You've coordinated with your team, OR
- This is your personal fork

The --force-with-lease flag is safer than --force because it fails if someone else pushed in the meantime.