how to keep iCloud’s photo backup and able to delete iPhone photos

VinTango Said:

"Thanks, I would like to keep copies in iCloud and not deleted in iCloud. Just delete local copies in iPhone."

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Upload to iCloud, then Delete form iPhone:

Then, upload these to iCloud. If you see a duly-grey down arrow, then that means it has not been downloaded to your iPhone. Once all of iCloud Photos is set up, it will upload: Set Up and Use iCloud Photos - Apple Support

Note: If there are hundreds or thousands of photos, then you'd need to upgrade your iCloud plan to iCloud+. Go here: iCloud+ - Apple

VinTango Said:

"how to keep iCloud’s photo backup and able to delete iPhone photos: I set to optimize iPhone storage already, does it mean I can deleted the copy on my iPhone while iCloud’s still have my photos copies"

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Back Up your Photos First:

All should be fine. If the photos are not downloaded to your iPhone, then they would have been deleted from iCloud.