Action 1:

Action essentials: stretch out an arm, straighten it out, and lift it forward from both sides of your body until your arm is shoulder-high and your palm is up. Holding the outstretched palm in the opposite direction with the other hand will make you feel the obvious stretching feeling of biceps brachii.

Action 2:

Action essentials: find a flat object, or a stretching frame, with your back to the stretching frame, extend your arm to stretch the biceps brachii, press your palm down on the plane, lean forward slightly, and squat down slowly, so that the biceps brachii will have obvious stretching feeling.