To change the duration or price of Nebula subscription, please follow these steps:
1. Open the Settings app.
2. Tap your name, then tap Subscriptions.
3. Tap the Nebula app to change a plan.
4. Tap See All Plans, then follow the onscreen instructions to complete the porocess.
Here is the guide from Apple: https://support.apple.com/en-us/HT204939