Q: how do I access the photos in icloud storage to delete some of them

If you have Settings > iCloud > Photos > iCloud Photo Library turned on and you delete any photo from your phone it will also be deleted from iCloud and from any other device connected to that iCloud Photo Library. There is no way to delete a photo from iCloud but have it remain on your phone (or vice versa).

You can use a computer and log in to https://www.icloud.com/ and go to the Photos app there. It will allow you to delete photos -- but they will also be deleted from your phone.