git rebase -i allows you to conveniently edit any previous commits, except for the root commit. The following commands show you how to do this manually.
# tag the old root, "git rev-list ..." will return the hash of first commit
git tag root `git rev-list HEAD | tail -1`
# switch to a new branch pointing at the first commit
git checkout -b new-root root
# make any edits and then commit them with:
git commit --amend
# check out the previous branch (i.e. master)
git checkout @{-1}
# replace old root with amended version
git rebase --onto new-root root
# you might encounter merge conflicts, fix any conflicts and continue with:
# git rebase --continue
# delete the branch "new-root"
git branch -d new-root
# delete the tag "root"
git tag -d root