I did this a long time ago on a LM2 with a Bosch alternator.
Take a picture of your alternator, it comes in useful when refitting and reconnecting the wiring.
Disconnect the wires and remove the alternator. Pull the brushes out of their ceramic holders being careful not to let the springs that hold them in, unravel.
Then de solder the old ones and re solder the new ones in their place. New brushes come with their own short wires pre attached, detach / reattach at the other end of this wire, not it's joint at the brush.
You may need to re use the wire conduit from your old brushes; it should be soft and flexible.
Clean up the rotor with fine glass paper and de grease it.
Reinsert the brushes in their ceramic housing with the springs.
When refitting the alternator, be careful that the brushes don't catch on the rotor as they are quite brittle.