Which rule prescribes the action to avoid collision by altering course and/or speed?

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The main idea is that preventing a close-quarters situation requires taking proactive changes in how you move. The rule that covers this tells every vessel to take early and substantial action to avoid collision, and that action is usually changing course and/or changing speed so the vessels won’t come too close. In practice, the vessel responsible for avoiding—often the one with the other vessel on its collision course—must initiate a maneuver well before danger develops, using a course change, speed change, or a combination of both to pass safely. The other vessel should maintain its course and speed unless it becomes clear that the first vessel isn’t taking adequate action, in which case it also must adjust to prevent a collision. This rule is the direct instruction on how to execute avoidance, whereas the other rules cover who has the right of way, what to do in special cases, and general lookout requirements.