Editing a point along a fixed axis?

I'm trying to take the solid triangle shape in Arkio VR using a Meta Quest Pro, and edit just the points of one vertex. If I use the orthogonal edit it moves the entire face (e.g. like an extrusion). When I use the freedom edit the whole side of the shape starts to move including vertices other than the single point I want to move. How do I shift the of the apex of the triangle/roof shape so that only the top of the shape is modified and the base of the shape remains the same?