1. Make the Tempo track visible.
2. [Assuming you already have the song at 120 bpm...] Locate the point where you want the tempo change to occur and place a point on the line.
3. Move a bit to the right and place a second point.
4. Grab the second point and drag it up to 140 bpm.
5. Finally, move the second point directly above the first point (you should now see the two points stacked, one above the other).
As long as there is no other point (a third point) to the right, everything beyond your change will be at 140 bpm.