How do I change the next leg of the course or deal with a missing mark?

Changing the next leg (rule 33). While boats are racing, the race committee may change a leg that begins at a rounding mark or gate by moving the next mark (or the finishing line) and signalling all boats before they begin the leg. The next mark need not be in position at that time. The signal is flag C with repetitive sounds, plus:

• for a change of direction: the new compass bearing, and/or a green triangle (change to starboard) or a red rectangle (change to port);
• for a change of length: a "-" to decrease or a "+" to increase the leg.
  Subsequent legs may be changed without further signalling to maintain the course shape.

Missing mark (rule 34). If a mark is missing or out of position while boats are racing, the race committee must, if possible, either move it to its correct position or replace it with a new one of similar appearance, or replace it with an object displaying flag M and make repetitive sounds.

Source: Racing Rules of Sailing 2025-2028, rules 33 and 34 and the race signals page, https://cdn.revolutionise.com.au/site/ofeyzh6cn3tumfga.pdf