There can be added two kind of feeds. Public feeds and a SharePoint feed.
First you have to give the new feed tab a name. This name will be displayed in the live tile.
The RSS feed URL can be retrieved from the website and/or SharePoint. Choose the Feed type matching your feed. And as last configure the Site URL. Where the "Open site" links to. To remove or add a tab press the + and -.
Optional is to change the order of the RSS tabs by clicking the arrow up or down.
As an administrator you can configure Predefined RSS feeds for your users. Go to Settings, App store and select the RSS tile. Here you can configure a Predefined RSS feed.
To make multiple RSS feeds click on the + icon.
When you are done click on "Add to workspace". Now you have you're RSS feed(s). If you have multiple RSS feeds and you want to switch, you can choose it with the Drop down menu.