How can I edit a published form already in use?

You can edit a published form the same way you edit a form you just created on the form's builder page.

Please know that changes you made to the form will automatically be saved and will be applied to the published form.

If your form already has existing submissions, please avoid deleting fields in your form that is associated with the submission data. Hiding the field instead would be the workaround — How-to-Hide-Form-Fields

If otherwise the changes you made are not applied to the published form, you can try clearing the cache of your form. How-to-Clear-your-Form-Cache