Upstrokes are essentially the opposite of downstrokes. Your hand is starting below the strings and playing up through them.
To play an upstroke, start your hand below the first string right beneath the 1st string. Quickly brush the pick up through the all of the strings. The main difference from the downstroke is that you're working a little against gravity for the upstroke and this requires a bit of arm and wrist movement. In the next lesson you'll learn how to combine down and upstrokes and how downstrokes can provide some momentum for your upstroke.