It is impossible to modify in PTC CREO a STL file opened as a part. To do some extra cuts on the STL file you have to:
Save the file as a copy:
![](https://www.rozenek.com/wp-content/uploads/2019/01/how-to-modify-stl-file-in-creo-01.png)
Then select shrinkwrap as a file type:
![](https://www.rozenek.com/wp-content/uploads/2019/01/how-to-modify-stl-file-in-creo-02.png)
In the dialog window increase quality to 10:
![](https://www.rozenek.com/wp-content/uploads/2019/01/image-1.png)
Save the file and enjoy editing.