how to edit step file in solidworks

STEP is a dumb format without history but you can add new features to the existing solid(s)

Editing a STEP (Standard for the Exchange of Product) file directly in SOLIDWORKS is not the most straightforward process because STEP files are typically used for data exchange between different CAD software and may not retain the full feature history of the original model.

However, you can try "Feature Recognition" option for Review or study the Model
may it's helps you?

Maybe you can try the "Direct Editing".

Open the STEP file in Solitworks, and then edit it/