If you have an active subscription, you need to cancel it first. You're able to cancel your paid plan here: https://www.yumpu.com/en/account/profile/edit#yp-current-plan
To delete your account visit the following link: https://www.yumpu.com/en/account/profile/edit#yp-delete-account
Attention: Your account and your documents will be deleted permanently and can not be restored.