depends on the Program you are using - normally ther is an option - export to 3d file - and ther you can select stl. take care that you are using the right units!
If you don't have the direct option for saving .STL you can save as an intermediate file like a .STEP, .IGES, or .IGS. Then you can open the intermediate file in a cad software from which you can save it as .STL.
I tried to export it as vrml and change it to wrl manually, then using blender to convert it to stl. However I cannot see anything in blender when I import the file and neither in Cura even if I export it as stl