Your opponent is in the mount, obviously you need a quick and reliable escape.
Grab your opponents right wrist with your right hand. Cup the hand with your thumb over the top - this make it harder for your opponent to pull their hand away.
Secure your opponents right arm by grabing their elbow with your left hand.
Take your left foot over the top of your opponents right ankle to secure it.
Raise your hips straight up then roll onto your left shoulder. Since you secured the right side of your opponents body, they have no way of bracing themselves to prevent the roll.
As you continue the movement you will end up in your opponents guard. Be sure to secure their arms straight away to prevent chokes and arm-locks. Although you are now in your opponents guard, it is a lot better to be here rather than in the mount.