I registered for a race. Can I go back and change parts of my registration?

So you signed up for a race. Now you need to change something.
The promoter of your event can can make changes to your class entries within the RaceReady system, or which bike / vehicle you plan to use, depending on their policy. RaceReady cannot make changes—it's in the hands of your promoter.
The reason RaceReady can't make changes is that events are complex—and each promoter plans them differently. Pushing changes to registration after it has already been made can affect promoters' plans in different ways, so we require them to make the changes themselves.
So, the answer is that you must contact your promoter, and request the change to your registration.