Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello !
If you add a Microsoft 365 subscription to an account that already has a Microsoft 365 subscription, the remaining time from your current subscription will be added to the new one.
You can learn more about this process here : https://support.microsoft.com/en-us/office/what-happens-if-i-add-another-microsoft-365-for-home-subscription-to-my-account-e53bde8b-ff38-414c-b2c3-63173a0668e0
Just redeem the offer on your Microsoft account : your current subscription will be extended accordingly.
Please let me know if you have any other question !