Replace fixed to variable potentiometer on DC regulator
To the right of the current knob is a knob to select voltage.  It is set to 6V in your picture.  Rotate that knob so that it points to &quot;3-15&quot; or whatever the right most marking is.  That is the variable voltage setting.
With the selector knob set to the variable range, the coarse and fine voltage knobs will set the output voltage.