Tap the pin icon to access this feature, it will turn blue. With the pin feature on, pinned tiles will remain on the desktop even when the Refresh/Clear button is tapped. To pin or unpin tiles the icon must be blue. Then simply drag the desired tiles to the desk to pin them. To unpin tiles, icon must be blue, then tap each tile. Or any tile may be manually deleted by tossing it up or pushing it off the edge.
