If you have access to SolidWorks you can simply import STEP product data (.stp, .step) to it and save as part file (.prt, .sldprt). Other than that, some 3rd party CAD conversion programs like the CrossManager or similar can also help you with stp to prt conversion.
Convert ISO-10303 STEP product data to CAD 3D part or component.
We have found 3 software records in our database eligible for .stp to .prt file format conversion.
Microsoft Windows software - convert stp to prt on Windows
A feature rich 3D CAD software by Siemens
Convert from stp file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned stp to prt. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A CAD converter with huge format support
A 2D and 3D CAD construction software