The instructions I’m not sure how to follow.

Hello dear friends, a couple days ago I installed open core and the automatically updated itself breaking the system. I was given these instructions and I was able to find a .pkg file containing the correct macOS, I’m just not sure how to execute the rest of the steps.

I just want my father to have his computer back because he really needs to do music on it and other things if anyone can help please let me know .