I'm afraid this will depend on how the site has been built as you're not restricted to where you edit a given piece of content such as a logo.

If the site is based on Alloy, the logo can be edited from the site settings tab. For other sites it would typically be alongside the site settings either on the start page or on a dedicated settings page, and for others it may not be editable in the CMS at all (i.e. hard coded in the views or CSS).

If you want to find out for sure you'll need to look at your layouts/views where the logo is added and see how it's being added.

As Paul indicates, the only way to know for sure is to

1. 'Inspect' the page using your browswer to identify where in the markup structure the Image is located.
2. Double check in the markup by 'viewing source' (ctrl-u) just in case the image is being loaded through some front end magic (unlikely but worth a quick check)
3. Start to look through your views in the solution. Locate the view that resolves to your page, and then step 'out' until you find the correct nesting of view that contains the same markup you saw in point (2)
4. You will then (hopefully) see some razor that will demonstrate to you where the logo is configured. If you are unlucky, you may even find that it isn't retrieved from an authorable property and has been 'hard-coded'. if this is the case, do the right thing and make it editable.

Hope that helps