The Lightning connector cable is stuck to the case with some adhesive. To detach the adhesive you will be sliding an opening pick between the cable and the case. Be very careful not to cut the Lightning connector cable itself.

Carefully insert an opening pick under the ribbon cable near the Lighting connector housing.

Slide the opening pick toward the logic board, but stop half an inch from the bend in the cable.

Continuing farther along the ribbon cable risks cutting it.