As both the IGES/IGS and STP/STEP are fairly common CAD formats used for exchange purposes, so it should be fairly easy to find a program that can convert one to another.
Convert ISO-10303 STEP product data to IGES 3D exchange format.
We found 22 software records in the database related to .stp to .igs file format conversion.
Microsoft Windows software - convert stp to igs on Windows
A 2D and 3D CAD construction software
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 igs. 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 PLM/CAD/CAM/CAE software suite
Leading 2D and 3D CAD development software from Autodesk
3D viewer and converter with huge file type support
A 3D modelling software
A CAD converter with huge format support
A 3D modelling application
A family of 2D/3D CAD software products for Windows
Apple macOS / Mac OS X software - convert stp to igs on OS X
Mac version of popular Autodesk AutoCAD software
Multiplatform software - convert stp to igs on any platform
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler