In the same way as normal exposure compensation, you can set the exposure compensation for the flash. The flash compensation amount can be set up to +/-3 stops in 1/3-stop increments. (If the camera's exposure compensation is in 1/2-stop increments, flash exposure compensation will be in 1/2-stop increments as well)
If flash exposure compensation has been set with the Speedlite and on the camera, the Speedlite's flash exposure compensation amount will override the camera's