There are 2 ways of changing your website logo. Overriding existing logo.png located in site_root/templates/template_name/images/style_name/ folder or uploading new logo trough template manager.

Overriding existing logo.png

To override existing logo.png file please open logo.psd ( requires Photoshop 7.0 or higher ) change it to your own preference.

Now replace existing logo.png file name in site_root/templates/template_name/images/style_name folder with new logo;

Go to your homepage and refresh. Your new logo should be visible.
Note: you might need to do "hard refresh" to clear existing browser cache for your logo image.

Change logo trough template manager

Go to template manager tab name Layout hover over existing logo than click on tool icon the logo settings panel will drop down.
Click on Select button and select or upload your new logo:

Select your new logo image and click on Insert button:

Your logo will be applied to logo preview image. Now click on Auto set params to automatically set the logo height and width based on the image size:

Click the Save button and go to your homepage and refresh..
Note: you might need to do "hard refresh" to clear existing browser cache for your logo image.