image in header

Sorry, sorry, didn’t mean to hijack anything. I’m new here …
Ok, my problem: Part of the header on our website is our business logo, but unfortunately it’s an old one and I’d like to replace it. I looked around in the template.php and header.php but couldn’t find the name of the old logo or URL anywhere, instead there’s another name for it which I cannot find in the media.
I’m too scared to delete this one and replace it with the name of our new logo that I uploaded in case I mess everything up.
Exactly in which part of the Editor do I find the logo and how do I get our new one on the website?