I inherited this website from my father-in-law. I need to make changes. I have no clue where to begin. I downloaded the Coffee Cup site design software but I have no clue what to do.

There are a number of videos from cc available that take you through step by step tutorials, I also do them but mine are more relevant and made specifically
to what you are wanting to do, the first one I do for people is a general looking around site designer where things are what things do,
Some users also find vitdeo conferencing useful especially for beginners and for specific problems .

There are others on this forum too who can and do that sort of stuff too
If you are interested you can contact me using the email link in my signature section below.

Welcome to the wonderful world of COFFEECUP

The next question(s) is did the original designer/owner of the website use site designer to make the website, and if so what version or do you not know that either.
Where is the website hosted as in which hosting service, have they been informed of the change and you have the log in details etc and the acces to the files on that server. Who owns the domain name has it been changed to you (assuming by inheriting that you mean it's been bequeathed)when does the domain name expire.
If the original website was created with something earlier than site designer, or indeed another application, then that's going to mean a complete rebuild not just a matter of editing the content.
All of these and more need to be addressed before you can even begin.