I assume you mean STEP?
.stl is a polygonal format, it is akin to the 3D version of a pdf file. Convenient, but a pain to edit.
STEP is a format that is based on solid modelling, not polygons. A step file may allow for polygons but unless your software is a solid type modeller or can export as such, you might be out of luck.
When I have an STL file and I have no access to the original files or STEP, sometimes I have to resort to re-modelling the part in an appropriate piece of software (I use Rhinoceros 3D).
It's a pain, but it's like trying to convert from a pdf file back to the original word document.....