It's not always possible to add additional files to your application once it has been submitted. However, some institutions allow additional materials to be uploaded online post-submission.
If this is the case for your the institution to which you are applying, that institution's Admissions office will configure a Checklist for your form. This Checklist might include a section to send the institution additional or missing materials. See if this is an option available for you by logging into your Activity Page.
If a Checklist is available, a View Checklist button appears within the View Details section for that form.
If no Checklist is available, or if the Checklist posted does not have a section to upload the necessary files, reach out to the institution's admissions office to see how they would like to receive these additional materials. Their contact information can be found on the institution's website.