How do I save updated data in a PHP form?

So I have a basic PHP form where users can come in, and update their info if they need to (textboxes php echo their data) if they click a button. How can I save what they updated and JUST what they updated? It's only one textbox at a time. I know it's done through Javascript, which is fine, but I am unclear on what exactly the code is. Plus, the code would have to be written into an onclick= for a button preferably (I have a "Save" button). Back end is MySQL, Apache, and front end is PHP/HTML. 20 years in IT and counting, baby \m/ > < \m/

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