Yes, you can edit your release while it's still processing, before it has been sent to stores.
Head to your Music dashboard and scroll down to Your Releases. Hover over the release you'd like to edit and hit the Edit icon button.
Please keep in mind that you will lose your place in the release processing queue if you start editing a release while it is being processed. You will have to resubmit your release through the checkout and may also have to change your release date.