If you sent the artwork, use the audit trail.
If you sent the item off for review, or if you're a project team member, you can find previous versions of artwork in the audit trail. To find the audit trail, go to the In Progress list and select your project.
Scroll down to the bottom of the project page and click the Blue Square button next to the heading for audit trail. You can use the audit trail to view download previous versions of work.
Any deliverables submitted for approval will show a blue arrow pointing rightwards. Click this arrow to go to the original file.
From this screen, you can see a thumbnail of that original deliverable, along with the responses you received. You may also review feedback or download the old artwork from here.
If you are a project team member without approval access, use the compare function on the project.
If you are a project team member, you can compare previous versions of artwork from the projects page. To find the compare button, go to the In Progress list and select your project. Then select the deliverable.
Looking at the current version of artwork, you'll notice that there's a Settings icon. Click the icon, then click Compare Revisions. You'll be able to toggle between the current version and any previous versions of artwork from here.