If you want to switch from a monthly to a yearly subscription to save some money here is how to do it.
Cancel your sub
Cancel your current subscription which prevents Marvin from billing you for the next month when this month ends.
Sign up for the yearly sub
Once your paid month has run out (you can tell because Marvin stopped working or it says so in your account page) you can sign up for the yearly sub.
If you sign up for the yearly plan before your previous paid month has run out, you will essentially pay double for those remaining days in the month.