/3/ - 3DCG

>>631271
Select the face
Ctrl + E
Two options. Rotate CW, and Rotate CCW. CW is Clockwise, CCW is Counter Clockwise.

Before I learned about this, what I would do is delete the face, extrude one of the vertexes which would create the needed edge, then merge that extruded vertex to the opposite end and fill the resulting tris.