You can do this via your account page.
Please log in, click the Library tab and then click "Change plan."
This will display your other options. (The payment currency will be the same as the currency of your current subscription.)
Select your new plan and the click "Submit."
If you have been migrated from the Lite plan to the Standard plan (launched Feb. 15, 2021), please note the following:
* If you cancel your subscription or otherwise break the subscription period, you will be charged the Standard plan price ($15/month, ¥1,500/month) if you resume your subscription.
* In addition, if you change to the Premium plan and then change back to the Standard plan, you will still be charged the Standard plan price ($15/month, ¥1,500/month).