The best way to change the formula of your subscription is to cancel your existing subscription to Silk or Silver, and re-subscribe the new formula.
To cancel the current subscription, you can follow the steps in the following link:
Once your current subscription is canceled, you can start a new re-subscription to NeuEve with the new formula.
Go to https://www.neueve.com/.
Scroll down to the Subscribe and Save section.
Click the yellow Subscribe button under the new formula.
Your change is done.