Quadrilateral plates are generally more accurate and efficient than triangular plates and so this tool lets you merge triangular plates into quads where possible. After selecting the plates to be merged you should right-click and then select "Mesh Tools" => "Merge Triangular Plates" from the popup menu that appears.
The tool merges as many triangular elements into quads as possible, however you should be aware that it is often not possible to merge them all as you can see in the "before" and "after" meshes below.
For further information about other plate mesh tools refer to "Mesh tools".