I assume you mean the largest dB/div dynamic range of the Return loss Format.
(The actual measurement range is limited in practice by the bridge directivity which varies with frequency)
Select MODE, and S11 to get into S11 mode
Select soft keys FORMAT then RETURN LOSS
Hit soft key dB/Div until the display is in 10dB/div mode (the largest range)
Press AMP key then turn Rotary Knob control (or use -/ keys) to move 0 to top of screen
Then press soft key EXIT and process with calibration or measurement.