How to remove synced photos

You have to sync again with any other device you have, to replace the previously synced photos by other photos.

You cannot remove selectively synced photos directly from your iPhone, you have to sync again.

When you sync again with another device, as you no longer have access to your PC, all previously synced photos will be roved, so you have to save them elsewhere to be able to restore the photos you want to keep.

Do you have another device available?

Plati_Q wrote: Why is it so complicated?

No one here knows Apple's thinking. But my take on it is that it's complicated because there are two incompatible systems:

Synchronization of Photos Libraries using iCloud Photos is new, easy, and automatic,

"Syncing," using a cable to attach a computer to an iPhone (which, by the way, is not really synchronization!) is the old system, and it's complicated mostly because it's older, harder, and less versatile. It's like using a phone with an old style mechanical dial instead of having buttons.

I remember how hard it was to call long distance with the old system.