Go to Imunify360 → Settings → Backups. Click Resize link. You will be redirected to the CloudLinux Network where you can add or remove storage space. 

After successful payment, the backup storage size will be increased. Imunify360 creates an initial backup of a current server if it was not done before or it just increases the storage size. On the Settings → Backups tab you can see the actual and used amount of backup storage in GB.