Modify .stl file

I am working with Slicer 4.11 and 4.13. I have a stl file and want to do some modification like cut a part and add another part to it. When I open the stl file from File tab as a data, all SegmentEditor tabs are inactive. Is it possible to modify stl file? If so how?
Many thanks in advance!

My stl file is Aorta with cannula connected.