They need to go into a lower orbit.
Work Step by Step
We know that Kinetic energy plus potential energy is constant. Thus, if the astronauts want to catch up to the space station by going faster than it, they need to go into a lower orbit, allowing them to go faster and thus allowing them to catch up to the space station. After all, by going into a lower orbit, potential energy decreases, allowing kinetic energy and thus velocity to increase.