Getting a repeat perscription is easy - just follow the instructions you were given and contact the suppliers as often as you need to keep yourself stocked with the items on your natural health Perscription.
If your Perscription changes at your next appointment it is unusual for an item to be changed as a matter of urgency - usually you can finish taking the item at the same, or a revised, dose until it is used up.
If you havent had an appointment for a while you may want to rebook and take the opportunity for an MOT health check, including a review of your current natural health perscription, to ensure everything is still relevant.