Just open your storage and press on the resource you would like to exchange for experience stars. You will see the icon with a star below. Tap on this icon if you want to exchange one object or hold it if you would like to exchange several. This feature is helpful if you would like to advance in the game faster or empty your storage.
Note: all the items from the "RARE" section are not kept in storage and can't be exchanged for experience. You don't need to worry about their quantity.