Take out the big silver screw on the front last, 1st go look behind the legshield and you will see 2 screws each side that screw thro to the front, take them out then go round the front and remove the big silver screw. That way the whole front panel with the headlight attached cokes off, pull the rubber protector off the main bulb and take it out.
If you hold the bulb up to a light you can see the filiments inside, most probably one of yours will be broken for the low beam. You can then take the bulb with you to a parts store and ask for a replacement or alternative.