Two ways ... in the main color selection box, lower down is lighter. But if you have the color and only want to change brightness, go to the number controls on the right side of the color selection box. You'll see a control or two there that are say % or something like that ... work with those.
In fact, you should just play around with the settable controls in that box to find out what they all do. You will want to know how and use them.