Default UI Menu: Modify/Modify 3D Objects/Surface Mesh/Merge Smesh

Ribbon UI Menu:

Merging is the opposite of refining. It takes all of the selected faces and creates one larger face. All of the faces must be contiguous (neighboring) and share edges, or they cannot be merged. The Edit tool Selector mode must be OFF or the Merge option will be unavailable. To merge faces, first select the desired faces.

Once all of the faces are selected, choose the Merge Facets option from the local menu or Inspector bar.