I've committed a bunch of commits to a project on Github, however I realized I hadn't set up the proper email and committer full name on the computer I'm currently using to make my commits and therefore the users avatar and email address are not there.

How can I rewrite all past commit email and usernames?

You can add this alias:

git config --global alias.change-commits '!'"f() { VAR=\$1; OLD=\$2; NEW=\$3; shift 3; git filter-branch --env-filter \"if [[ \\\"\$\`echo \$VAR\`\\\" = '\$OLD' ]]; then export \$VAR='\$NEW'; fi\" \$@; }; f"

To change the author name:

git change-commits GIT_AUTHOR_NAME "old name" "new name"

or the email for only the last 10 commits:

git change-commits GIT_AUTHOR_EMAIL "[email protected]" "[email protected]" HEAD~10..HEAD

Alias:

change-commits="!f() { VAR=$1; OLD=$2; NEW=$3; shift 3; git filter-branch --env-filter \"if [[ \\\"$`echo $VAR`\\\" = '$OLD' ]]; then export $VAR='$NEW'; fi\" \$@; }; f"

Source: https://github.com/brauliobo/gitconfig/blob/master/configs/.gitconfig