If you've never used DW before, there are a few things you need to do to make updates to your current site.
You will need to define a site on your computer first, including adding the FTP info for your current site: https://helpx.adobe.com/dreamweaver/how-to/define-a-dreamweaver-site.html
From there, you can download your files from your remote server by hitting the "Get" button (down arrow icon in the Files window).
You then work on the local files to make changes, save and upload them with the "Put" command (up arrow) overwriting the file on the server.
If your site is currently database driven, the above won't work without also making a copy of the database itself and setting up a local testing server with the same type of set-up as your current remote server.
It might help a bit if we could see what you're working with. Can you post the URL to your current web site?