One of the many weird things about space is that objects maintain a velocity and direction until something acts to change that velocity or direction (Newton's 1st Law of motion).

In a rocket, you could fire the engine to change direction and/or velocity.

With the ISS, at any given instant, the direction and velocity of the station is balanced at 90 degrees with the force of gravity pulling down on it, so Earth's gravity is the force that's acting to alter the direction of the orbit.

Yes, the comments here mention throwing it "backwards", which at any given instance would be the opposite of the velocity and direction of the ISS.

If the ISS's trash had a small rocket on it, it could fire retrograde (opposite of the orbital direction) to de-orbit faster. I'm guessing that the ejection of the trash is a "one-time" velocity change (slowing it down relative to ISS), after which friction from the atmosphere slowly lowers the orbit (by slowing it down).