Currently I am converting an AutoCAD Draw project (DXF file) to QGIS where I have some several shapefiles as skeletons to combine with data. (Cut from one layer and paste them into a another. I have converted polylines (which are contours) to polygons. This is because "normally" these polylines are supposed to be closed. However, I encountered a problem. Two specific polygons which are supposed to be the same one, have not been converted correctly for unknown reasons.
This what is supposed to look like. The black lines is suppose to be closed and go around the square object, this is what was not drawn that way. But you will get the point.
How do I merge them together?