Use a CAD program such as FreeCAD, SolidWorks, Fusion 360, AutoCAD, CATIA, or Creo
Open the program
Click File
Select Open
Choose the STEP file with the .step or .stp extension
Confirm the import if prompted
View the model in the workspace
Use a STEP viewer such as eDrawings, Autodesk Viewer, or FreeCAD if you only need to inspect the file
Convert the STEP file to another format if your software cannot open it directly