Your Weapon level is tracked on the board. You can increase your Weapon level through Build Actions assigned to the Weapon level track.
You may also gain Items or other game effects which offer you a temporary Weapon level increase. Temporary increases are for the purposes of one combat, requirement, or other situation, and are then cancelled. They are not tracked on the board.
If you are instructed to reduce your Weapon level, this reduction is always made on the track on the board, never applied to temporary Weapon level.