The option to export a model as an STL file in Fusion 360  is hidden. To export your model as an STL, do the following:

1. Load your project and Right Click on the Part you wish to Export.
   
    
2. Select Save as STL.
   
    
3. Make any final changes in the Panel then click OK when you're happy with the settings. Select the Location you wish to save the STL to and Click Save.