As both the IGES 3D Exchange format (.igs, .iges) and ISO STEP product data format (.stp, .step) are somewhat common and usially well-support CAD formats used for exchange purposes, it should be easy to find a program that can convert one to another or vice versa.
Open IGS file Open STEP fileA CAD viewer program with conversion capabilities
A set of 3D modeling tools
A 3D modelling software
A converter for 3D formats
A CAD conversion software
A CAD converter with huge format support
A CAD visualizaton, analyzation and translating system
An open source multiplatform 3D CAD/MCAD/CAx/CAE/PLM modeler
A suite of CAD applications
A dedicated CAD converter