How do elevons work to roll a flying wing?
Good question! There's a bit of a misconception: when the elevon moves up, it actually decreases lift. It pushes air up which pushes the wing down.

This explains the roll behaviour, but how does decreasing the lift make the plane go up? The key here is that the lift is reduced only at the rear of the plane. In other words, the rear of the plane is pushed down, but only the rear. This makes the nose point up (just put the plane on the floor and push down on the tail, the nose will be up).

In this new orientation, the angle of attack (the angle of the wing relative to the airflow) is increased, and consequently the lift increases (the little bit of downforce at the rear of the wing doesn't really make a big difference). The plane will start to climb, which will reduce the angle of attack (the wing will be pointed the same way as the direction of travel), until the plane is in a steady climb.