Deleting images from Google Photos without deleting them from the device
You can go through the web interface https://photos.google.com/, download all your photos in order to have a backup, then you can delete the needed ones.
In case the deleted Google Photos from the web interface delete also the ones in the device, you can restore them to it using USB connection (Folder: Internal Storage/DCIM or External Storage/DCIM if you have a SD card).