For most flashes, flash exposure compensation (FEC) can be set by pressing and/or holding the [SEL] button until the FEC icon is flashing in the display.
Then press the [+] or [-] or turn the dial to select the chosen FEC.
the FEC adjustment will be displayed on the screen. Then press the [SEL] button to confirm your selection.
Be sure to set the FEC back to zero when not using a diffuser.
If this does not work for your particular flash, please consult the instruction manual.