What are the rules for sailing instructions and changing them?

Under rule 90.2 the race committee must publish written sailing instructions that conform to rule J2 (Appendix J).

• Changes must be in writing and posted on the official notice board before the time stated in the sailing instructions, or, on the water, communicated to each boat before her warning signal.
• Oral changes may be given only on the water, and only if the procedure is stated in the sailing instructions.
• For an event where entries from other countries are expected, the sailing instructions should, when appropriate, include the applicable national prescriptions in English.

Source: Racing Rules of Sailing 2025-2028 with Australian Sailing prescriptions, rule 90.2, https://cdn.revolutionise.com.au/site/ofeyzh6cn3tumfga.pdf