Navigate to the Control Panel > Website > Website Modules > Document Library. Find the folder where the document is located, right-click it and select View Documents.
Click the Edit icon. Then, check the box "Update".
This option allows you to refresh a document already uploaded. Browse to the document on your local hard disk or network and select it. By default, the system preserves the original filename, even if the updated document has a new filename, overwriting the old version of the document. But you can also tell the system to use the new filename and the original document will be deleted.