In the envelope graphic, you can see various handles (junction points) of two different sizes. Drag these handles to adjust the envelope shape.
Both of the larger handles on the x-axis (the horizontal, or time axis, at the bottom) control the attack and decay times, respectively. A vertical line extends up from the first of the two handles (attack), and divides the envelope into an attack and decay phase.
Both segments have two small curve handles. You can drag these in any direction to deform the contour of the envelope and shape its amplitude.
You can also directly drag anywhere on the curve itself to reshape the envelope.
Download the guides:
Logic Pro User Guide: PDF