You can do it easily through your own account.
By clicking "Change Selections" you are moved to a screen where you can edit your selections. After selecting your new preferences, you still need to move to checkout to confirm your selections.
Switching your subscription is for free, unless you change the coffee bag size to a bigger one.
Please note, that changes to your subscription will always apply to coffees from the next renewal forward, because earlier coffees are already in production.