ref:

The problem with the other two answers is that the new master doesn't have the old master as an ancestor, so when you push it, everyone else will get messed up. This is what you want to do:

  git checkout better_branch
  git merge --strategy=ours master    # keep the content of this branch, but record a merge
  git checkout master
  git merge better_branch             # fast-forward master up to the merge


If you want your history to be a little clearer, I'd recommend adding some information to the merge commit message to make it clear what you've done. Change the second line to:

  git merge --strategy=ours --no-commit master
  git commit          # add information to the template merge message