If you want to change a rule for an event, RRS 86 explains which rules can be changed and by whom. There are very few rules that can be changed by the Sailing Instructions - and none of the rules in Part 2 (the "right of way rules") can be changed by the sailing instructions.

If you're referring to actually changing a rule in the rule book, the process is long and somewhat arduous. Basically, an MNA (like US Sailing), an International Class Association (like the Laser Class), or an internal committee of the ISAF makes a submission to the ISAF Rules Committee. They will ponder it, perhaps propose some modifications, and then recommend to the ISAF Council whether to accept or reject it. The ISAF Council will vote on it, and if approved, the submission will be incorporated into the official Racing Rules of Sailing.

In this, the 3rd year of the 4 year rules cycle, most proposed changes have progressed to this last step. There were literally hundreds of submissions processed by the Rules Committee during the ISAF Conference last November in Puerto Rico. You can read the minutes here. If you want to get a feel for what's coming up in the next rule book. Copies of the submissions are found here.

Most of the rule changes recommended by the Rule Committee are not "game changers". There's some clarifications, and a re-organization of rules 19 and 20 to have them conform logically with rule 18.