The easiest way to change the logo is to replace the existing one via FTP; if you don't have FTP access, the other option is to use a widget in the "Logo" position:
1. If you uploaded your logo using the WP media uploader, go to your media library, "Edit" the logo item, and copy the file URL (shown on the right bar of the media editor).
2. On the Appearance->Widgets screen, place a Clean Markup or Text widget in the Logo position and paste in the following code (after editing it to use the URL for your image):
<img src="http://url.to/your-image.png" />
3. If you want to have the logo clickable as a link back to your site homepage (common practice for most sites now), use (and edit) the following code instead:
<a href="http://url.to/yoursite"><img src="http://url.to/your-image.png" /></a>