How do I export a linework layer with an 'altitude' attribute in QGIS to a DXF with lines raised in the z according to their altitude attribute?

I have contours which I have brought into QGIS from a GEOjson - the lines have an attribute of 'altitude'. I want to export these contour lines as a DXF with the lines raised in the zplane according to their 'altitude'. I'm new to Qgis - have heard I can use the 'command line' (Is this the 'commander' under the processing tab?) which suggested org2org?? but I couldn't understand how to use this at all. Any help would be appreciated.