If you are using our default template, there are two ways to change the logo. There are :
Overwrite Logo Images:
By default all logo and images are located at /<$ServerPath> /application/layouts/scripts/<$TemplatePath>/images/ therefore you can upload & overwrite logo image to application/layouts/scripts/<$DefaultTemplateDirectory>/images/logo.png
Update the Header Block:
By default all design blocks are located at /<$ServerPath> /application/layouts/scripts/<$ DefaultTemplateDirectory >/blocks/ therefore you can edit top-menu.phtml page to add custom logo location and save it.