Flip Face Edge

The Flip Face Edge action swaps the triangle orientation on the selected face(s) with four sides. This reverses the direction of the middle edge in a quad.

Use this to smooth ridges in quads with varied height corners.

Note: This only works on quads (four-sided polygons).

This action is often called Turn Edges in other 3D modeling applications.

From the main menu, select Tools > ProBuilder > Geometry > Flip Face Edge.