Browse to manage.autodesk.com and log on with your Autodesk ID that is linked to your contract. All your software, including previous versions and serial numbers will be listed on the Products Tab, along with download links.
(your Autodesk ID will have been sent to you from Autodesk when your subscription contract was set up. If you don't have this, contact your re-seller)
You can download almost all Autodesk software from the Virtual Agent including previous versions.
Click on Download Links then follow you nose through the product categories. If you are after a suite (e.g. Autodesk Building Design Suite), click on Autodesk Suites.