I launched my website from a software house, How I can edit the source code of my published website?

Hi Everyone, Actually, some years before, I launched my website from a software house, Now I am learning SEO(Search Engine Optimization), And I need to make some changes in the source code and also wants to save them on the server as well, But I don't have source code, do I need to contact the software house to get source code, or there any tools available? Thanks.