You can change your plan by asking for it to be reviewed. This is when someone checks your plan to see if it still suits your needs.
Your plan is reviewed every 12 months. But if you're unhappy with your plan, or there has been a big change in your life you can ask for it to happen earlier. If it's only something small, the NDIS can make a light touch review, which is a small change, to your plan.
You can tell the NDIS you need a review with the form at the Change of Circumstance page: https://www.ndis.gov.au/participants/understanding-your-plan-and-supports/change-circumstances.