Hi @BeauSanders and a belated welcome to the phpList forum!
The answer is it depends what part of the subscribe page you’re referring to. The input fields and their labels are generated automatically depending on the configuration options for the subscribe page that you choose. That is designed for convenience, but it makes it difficult to change how they are displayed. To do so you’d need to look at the logic probably of index.php in this folder (I may be misremembering the, root around).
The other parts of the page should be easily editable by changing the HTML on the settings page either for the subscribe page in question, or on the settings page of your installation (that changes defaults for all future subscribe pages).
In case it’s helpful to know, The HTML is stored in the database and theoretically you can edit it directly there, though there should not be any need to do that. Already jQuery is included on subscribe pages, and in a recent update of phpList the the default HTML was changed by updating the default SQL that is used for those.
If you edit the logic of the pages then please share a patch here or in a pull request on GitHub, if appropriate, so that others may benefit too. Thanks!