Image Credit: https://commons.wikimedia.org/wiki/File%3AEdith_McGuire_1964.jpg
You don't need to be a registered user to edit Wikipedia. Anyone can edit an unprotected page by clicking the "Edit" tab at the top. If you edit a Wikipedia page while you are not logged in, any edits you make will be attributed to your IP address.
Creating a user account does have some benefits that you should consider.