Cancel the membership. It will run until the next renewal date, and then stop. Wait for the payment date you want in future to come round, and resubscribe on that date.
If that will mean that you are without your membership for too long a period, another approach is open a new Now account and start a membership on the date that you want your payments to come out on. Then cancel the original membership.
This approach does mean there is an initial time period when you are effectively paying twice, so you need to choose between this and the first approach, where you have a time period in which you can’t access your membership. So the answer may depend on how long you want to fund an overlap for, or how long you can bear to be without your membership.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.