Lets suppose my program makes a http request and uses it response to update some values. Is it possible to somehow mimic the response in order to modify it so the program believes the server responded when in fact it was a hardcoded response? I'm not using HTTPS.
Yes, it's possible and it's trivially easy.
You can make it a bit harder by using HTTPS and a pinned certificate but that will not stop anyone with a bit of time on their hand (they will then need to modify your executable, or, at least, the container where you placed your pinned certificate) and it will make it harder to manage your server (because if you need to change your certificate, you'll need to update your clients as well).