You can't update feature geometry from the Field Calculator. Instead use the Geometry by expression tool (find it in the Processing Toolbox).
"This algorithm updates existing geometries (or creates new geometries) for input features by use of a QGIS expression. This allows complex geometry modifications which can utilize all the flexibility of the QGIS expression engine to manipulate and create geometries for output features." - excerpt from the tool description in QGIS
geom_from_wkt("WKT_path"), should work in this tool, as long as the layer has a field called "WKT_path" with the new geometries in WKT format.