How to edit imported STEP files

Hi,

I have imported a STEP file successfully but I can't seem to edit it. In the part manager I can select individual faces/shapes but when I try to delete them the whole part deletes.

Any ideas what I am doing wrong?

Thanks

https://cad.onshape.com/documents/92abb5e64eb4bc3d6cf02373/w/6feef26855f4d727a7467fdd/e/0b82c1d642723d7c6eb89d8e?renderMode=0&uiState=62dbff72aa0f1f435fbc7839