Type "flip" into the effects panel search bar.
3D perspective can do this also.
Be careful... this may not necessarily fix a "line cross". There are others factors that get flopped that can make the flop not work. eg left handed vs right hand, visual continuity of things such as hairstyle, pockets, face shape etc....
Thanks to both of you--now that I know I can "flip", I see that by so doing I can also "flop."