Hi, I'm Bernard. I'm happy to help!
To renew the subscription, kindly follow the steps below:
Go to this link: https://www.minecraft.net/en-us/login and sign in to your account.
Click on Realms.
Find the Realm you want to reactivate under Inactive Realms.
Click the Reactivate button to renew your Realm subscription.
For reference: https://help.minecraft.net/hc/en-us/articles/4410000696077-Minecraft-Java-Edition-Realms-Billing-Issues-FAQ