I am curious about how to remove the first commit in git.
What is the revision before committing any thing? Does this revision have a name or tag?
For me, the most secure way is to use the update-ref command:
git update-ref -d HEAD
It will delete the named reference HEAD, so it will reset (softly, you will not lose your work) ALL your commits of your current branch.
If what you want is to merge the first commit with the second one, you can use the rebase command:
git rebase -i --root
A last way could be to create an orphan branch, a branch with the same content but without any commit history, and commit your new content on it:
git checkout --orphan <new-branch-name>