Microsoft Helpline 1-877-424-6647
Firstly check your auto renew setting by signing in to www.office.com/myaccount. Look under "Payment and billing" to see if automatic renewal is set to "On." If automatic renewal is set to "On," your subscription will renew automatically. If automatic renewal is set to "Off," go here and select the Office 365 version you want to renew.
If the same MS account that you used when setting up to your original subscription then any time remaining will be automatically added to your existing subscription. If you are using other MS Account then you will create a new subscription.