Beste antwoord door Stijn NS
Good afternoon Chris23 and welcome to the NS Community!
The beauty of the NS Flex subscription is that you can change or cancel it on a daily basis once the first month (and any other 'cooling down' months) have passed. You started the subscription on 10.12.2018, which means that you can change your subscription on a daily basis from 10.01.2019 on. Which means you can simply cancel the subscription on 20.01.2019.
As for the costs, you'll pay any subscription costs in advance for the coming month with the invoice that you'll receive on the 10th of January, but you'll get a refund for the period you've not used on the next invoice. So on the invoice you'll receive on the 10th of February, you'll get a refund for the period 20.01 - 09.02 that you've paid in advance, but didn't use.