You can delete synced photos from your iPhone by turning off iCloud Photos or, if they were synced from a computer, by changing your sync settings on that computer. The method depends on how the photos were originally synced to your device.

If your photos are synced with iCloud

This is the most common scenario. When iCloud Photos is enabled, any photo you delete from your iPhone is also deleted from iCloud and your other devices. To delete photos only from your iPhone, you must first disable iCloud Photos.

Go to Settings. Tap on your name at the top, then tap on iCloud > Photos.
Turn off iCloud Photos. Tap the toggle next to “Sync this iPhone” (or “iCloud Photos” on older versions) to turn it off.
Choose “Remove from iPhone.” A pop-up will appear asking what you want to do. Select Remove from iPhone to delete all the synced photos from your device while keeping the originals safe in iCloud.
Confirm deletion. Open the Photos app, scroll down to the Utilities section, and open the Recently Deleted album. Tap Select, then Delete All to permanently remove the photos from your device.

If your photos were synced from a Mac or PC

Photos synced to your iPhone from a computer via Finder (on Mac) or iTunes (on PC) cannot be deleted directly from your phone. You must use the same computer to remove them.

On a Mac

Connect your iPhone to your computer with a USB cable.
Open Finder and select your iPhone from the sidebar.
Click the Photos tab at the top.
Change your sync selection. In the “Sync photos to your device from” menu, select the option to sync from a different (preferably empty) folder or simply uncheck “Sync Photos” entirely.
Click Apply. A prompt will appear asking if you want to remove the photos. Confirm the action to delete the synced photos from your iPhone.

On a Windows PC (using iTunes)

Connect your iPhone to your computer with a USB cable and open iTunes.
Click the device icon in the upper-left corner and go to the Photos tab.
Turn off photo syncing. Uncheck the box next to “Sync Photos” and click Apply. When prompted, confirm that you want to remove the photos.