Here is an article that can help you address this backup issue:
If your iCloud storage reaches its maximum capacity, you can take any of the following actions:
- Purchase more storage.
- Create a smaller backup. Tips for creating a smaller backup:
- Select which apps to back up. Include only the apps that contain important data.
- Reduce the size of your Camera Roll backup by deleting unwanted photos and videos before backing up. Optionally save the photos and videos to your computer before deleting them.
- Turn off backup for Camera Roll, especially if you already back up your Camera Roll to your computer. (Your iCloud Photo Stream is not a backup of your Camera Roll.)
- Delete old messages from your device, especially messages with attachments. To delete a message, swipe over it, then tap Delete.
- Delete items that you no longer need from iCloud, optionally saving them to your computer before deleting them. For example, you can:
Take care, and thanks for visiting the Apple Support Communities.