You can change from a month-to-month service to a 12-month contract, but not vice versa. The only plan change available for a 12-month contract is to move up in value to another 12-month contract, but this will also reset the 12-month contract period starting at the time of the plan change.
Importantly, plan changes can only take effect at the start of your next billing cycle.
Please let us know what plan you would like to upgrade or downgrade to and we'll schedule the change for you if possible. After that, it's just a matter of waiting for your next billing cycle for the change to take effect.