How can I change logo?

In the old templates, especially the old templates for joomla 1.5 the logo is placed in the template's directory.
To change logo in that template, navigate to template's directory templates/your-template-name/images/ and find image file named as logo e.g. logo.png
Then replace it with your image.

Another info about logo replacement you can find in the "Template Features" article under "Template Parameters" title, for example JM-Fashion-Catalog
Navigate Extensions->Template Manager and click on the proper template name to enter/select the logo file location.