@azarek At this point, I believe you've already paid the full amount for until August.
If your goal is to switch to monthly once August comes around, the simplest way is to click "Cancel my membership" (or similar wording) in your account. After a few seconds, the message should switch to "Canceled (ending August XX, 2019)" (or similar wording). Then after that date in August comes around, just get a membership again with monthly.
If you're trying to get a refund for what you've already paid, then I'm not sure what JNC's policy is and whether that's possible or not. But either if it was possible it would likely require an email.