In order to enroll in the updated version of the Nanodegree program, you should first follow the steps to ‘Cancel’ your enrollment in the current version, which can be found here. Please note that refund eligibility will be determined as per Udacity's existing policy.
Once you have successfully canceled your current enrollment, you can enroll in the new version via the program overview page.
It's important to note that, if you switch from one version of the Nanodegree program to another, it's unlikely that your progress in the program will be retained. See the FAQ "Will my progress in the current version be able to transfer over to the new version?".