While viewing your Gallery folders, tap (in the upper-right corner of
the screen) and select the photos and videos you want to delete, then
tap DELETE (at the bottom of the screen), then
tap YES to confirm.
While viewing a photo or video, tap the Delete icon (in the upper-right
corner of the screen) to delete. After deleting, you will see UNDO (at the bottom of the screen) for a while. Tap UNDO to restore the deleted file.