A lessons-learned review is a process of evaluating the effectiveness and efficiency of the incident response plan after an incident or an exercise. The purpose of the review is to identify the strengths and weaknesses of the incident response plan, and to update it accordingly to improve the future performance and resilience of the organization. Therefore, the incident response plan should be updated after a lessons-learned review.
References: The answer was based on the NCSC CAF guidance from the National Cyber Security Centre, which states: "You should use post-incident and post-exercise reviews to actively reduce the risks associated with the same, or similar, incidents happening in future. Lessons learned can inform any aspect of your cyber security, including: System configuration Security monitoring and reporting Investigation procedures Containment/recovery strategies"