Change content after data has been entered into the database

I have a page where the user enters his email address to subscribe to a newsletter. This is then forwarded to a PHP file via an action tag in the form, which then saves it in a database. The page will then be updated.

Now I would like to give the user some feedback that everything went well.

Above the button there is a text with an id = notify- It says something like: Subscribe to our newsletter. Now I want this text to be replaced with something new, but with the same CSS formatting. How do I do that ?