Unsubscribe nana Premium can't be proceed from App. Please check below.
【iOS】
iOS device settings＞Tap your name＞Subscriptions＞Cancel Subscriptions
*For details, please check Apple support page. https://support.apple.com/ja-jp/HT202039
【Android】
Android device settings＞Google Play Store＞Menu＞Subscriptions＞Cancel Subscriptions
*Even if unsubscribe nana Premium, you can use nana Premium till expire date.
*Notice that uninstall nana or delete nana account doesnt' mean unsubscribe nana Premium.