Are you having trouble uploading a file to your submission that has been opened for editing? Sign in to your Submittable account and click on the title of the submission that has been opened for editing (the row will be highlighted with a light blue background).
Scroll down to locate the File Upload section of your submission. Click the Add a file button on the right to upload your file.
When you are finished editing your submission, click the Mark as Done and Close For Editing button.