How to delete photos from iphone 6s without deleting from icloud

Not possible. When you delete from your phone, iCloud will update to reflect the changes. Delete from one, delete from all. Set up and use iCloud Photos – Apple Support (UK)

Make sure you make a backup and upload all photos to icloud. Once that is done- sign out of i cloud. Whatever pictures you have on your device will appear, that is when you select download and keep originals it keeps originals in the phone memory. Once you have signed out you can delete the photos saved on your phone. Then you can check your iphone storage before you sign back in to icloud.