The deadline for IAC Rules Change proposals is July 1, as noted in the Forward to the IAC Contest Rules. If you're planning to propose a rules change, the best way to do so is to go to this address: https://www.iac.org/propose-rule-change, fill out the form, and hit "Submit". Login to IAC.org is required.
If you happen to be an IAC member but for whatever reason can't use the above link, you can send a proposed change to ruleschair [at] iac.org and it will be forwarded to Doug Sowder. If you go "formless", please include your IAC number and expiration date, a synopsis of your proposal, the rule number, proposed new text, and rationale for your proposed change. Please be aware that a change to any rule can cascade through many other paragraphs of the rule book, so include any collateral effects that you're aware of.