To decrease storage on a project, you will need to raise a support ticket for a Support Engineer to review and perform the downsize. This is to ensure that data on the project is not at risk of accidental deletion.
In order to perform a storage downsize for Grid projects, the total storage allocation of the "disk" variables within the ".platform.app.yaml" and "services.yaml" files must be below the requested project storage change. Each environment should be calculated separately, as the project storage availability is the amount available per environment.
Please note that direct storage downsizes are not currently supported for Dedicated environments.