If you have a multiball game, you have several options. You can enter the service menu (consult your manual), then enter the tests and have the machine eject the pinballs. Some machines have a menu item titled "Trough Clear". But most modern machine only have a solenoid test which will do the same thing. Enter the solenoid test and let it run until the pinballs are ejected. you may have to close the coin door for the solenoids to work.
Alternately, you may be able to manually move the pinball eject mechanism to move the pinballs to the plunger area. For some machines, you may only need to move one solenoid (as shown). For others, you may have to move one solenoid to position the ball, then a second one to eject the ball.
The picture to the right shows the mechanism used on Bally / Williams games made in the 90's to eject the pinballs to the plunger area. A quick push upon the "C" shaped metal piece will eject the pinball.
After you have gone to all this trouble to remove the pinballs, go ahead and treat your machine to a new set. New pinballs will reduce the wear and tear on the playfield.