You can create a Blue/Green deployment. This allows you to make the change with just a few clicks in the Console and without modifying your application.

Steps:

1. Create a new RDS Blue/Green Deployment with the desired size
2. Validate the it has the correct size
3. Switch over to it
4. Delete the old instance
5. Delete the Blue/Green Deployment

https://aws.amazon.com/about-aws/whats-new/2024/11/amazon-rds-blue-green-deployments-storage-volume-shrink/

If you encounter an issue where the creation doesn't reduce the size, you might be trying to shrink it too much. Try a larger value.

Thanks to Sean for pointing this out in a comment.