Do you mean the feature light or the backlight for the display? The feature light can be set to on or off.
To set the light, press Save then 1 (for light on) or zero (light off). Press the desired memory key to associate with the feature and then enter the code from your telephone provider for that feature. Press Save and then label the memory key.
To use the feature light, press the memory key (as selected above) and then lift the handset. Press either Handsfree or Mute. The feature and the light will turn on or off. When the phone is not in use, then Feature On will be seen in the display.
The backlight is not adjustable. You can adjust the contrast with the Contrast button. Backlights do fail over time (but it's not a repair I recommend if the phone is out of warranty).
The manual is available here: http://www.aastratelecom.com/cps/rde/xchg/en-ca/hs.xsl/21564.htm
. Look at the bottom of the center section for the PDF.
I hope this helps.