You can adjust the flash output as easily as normal exposure compensation. Flash exposure compensation can be setup to +/- 3 stops in 1/3-stop increments. (If the camera's exposure compensation is set in 1/2-stop increments, flash exposure compensation is set in 1/2-stop increments)
If the flash exposure compensation has been set with the Speedlite and the camera, the Speedlite's flash exposure compensation amount will override the camera's
Using <>
This is for setting various numeric settings. Pressing the <> (plus) button increases the number, while the <> (minus) button decreases the number.