Hello, I've just started using it and already find that speckit is a highly impressive project，my question is how should i renew my plan?

I have a question: if I have initiated a specification and progressed to the end of the implementation phase, but suddenly realize that a feature was either not implemented or implemented incorrectly during the planning stage, what should I do now to update this task? I tested using /plan, and speckit completely rewrote my plan, which is not what I wanted.