SolidWorks supports STEP product format (.stp, .step) by default and thus is the obvious choice for sldprt to stp conversion.
If you do not have access to SolidWorks, you have to rely on some third party alternative with support of SolidWorks formats. STEP format is nowadays well-supported so that should not be an issue.
Open SLDPRT file Open STP fileA CAD viewer program with conversion capabilities
A set of 3D modeling tools
3D CAD/CAM tool for product development
A CAD converter with huge format support
A CAD visualizaton, analyzation and translating system
A dedicated CAD converter