Total newb question, how do I edit the source code of an individal post?

I recently set up my first wordpress website, with a free theme which I found online (so, no support).

Basically, I want to be able to attach an affiliate link to the featured picture (which is part of the theme, you just enter the link of the image and it features it), but how do I actually ACCESS the post source code in order to edit it? I can access the content of the post (text) using the mySQL database, but I can’t edit the code?

Apologies in advance for being stupid.