Use git-filter-branch.

git filter-branch --commit-filter 'if [ "$GIT_AUTHOR_NAME" = "Josh Lee" ];
  then export GIT_AUTHOR_NAME="Hobo Bob"; export GIT_AUTHOR_EMAIL=hobo@example.com;
  fi; git commit-tree "$@"'

This only affects the author, not the committer (which for most commits will be the same as the author). If you want to rewrite those as well, set the GIT_COMMITTER_NAME and GIT_COMMITTER_EMAIL variables.

The standard warning about rewriting history applies; only do it to history that has not yet been shared.

June 2018 Update

The manual now includes a solution, using --env-filter, in its examples: https://git-scm.com/docs/git-filter-branch#_examples :

git filter-branch --env-filter '
    if test "$GIT_AUTHOR_EMAIL" = "root@localhost"
    then
        GIT_AUTHOR_EMAIL=john@example.com
    fi
    if test "$GIT_COMMITTER_EMAIL" = "root@localhost"
    then
        GIT_COMMITTER_EMAIL=john@example.com
    fi
' -- --all