Project -> Properties -> Java Build Path
Click "Add Library..." button.
Select "JSF Libraries" and click "Next >" button.
Select desired JSF Library and click "Finish" button.
Project -> Properties -> Java Build Path
Select desired JSF Library and click "Remove" button.
To control which JSF Libraries are deployed to the web application and which are not, use the "J2EE Module Dependencies" functionality.
Project -> Properties -> J2EE Module Dependencies
Select/deselect JSF Libraries as desired (selected means deploy, deselected means do not deploy).