| You raise the action by adjusting the height of the saddles at the bridge. This is typically done by taking an allan key and simply turning the screws of the saddles clockwise to raise the strings further off the fretboard.
After doing that, be sure to check your intonation and adjust it as needed to keep the strings in tune on the higher frets.
Just a question, but why do you want to do this? Are the strings buzzing on the frets? It could have more to do with the truss rod - controlling the neck relief - than the action of the bass.
Hope this helps.
Craig | 
