To view all your previous Certificates, use the My Dashboard link in the top navigation menu, then select the Certificates tab.
On the Certificates page, you'll see the most recent certificate you've earned per Course. If you want to see all Certificates earned (even multiple times per Course), you can select the All tab.
Click individual items to see expanded details, including a link that will allow you to download the certificate. The Download All link in the upper-right corner of the page will download a .zip file of all Certificates you've earned for all time.