that logo is called favicon. You can change it in two ways.
1- public_html/templates/ja_opal inside there, you will find favicon.ico. From here, you will need to create your own with same size (usually 16×16 or 32×32). Call it “favicon”. Upload it so that it can overwirte the existing one.
2- download this component. When you finish installing the component, it will automatically create a folder in public_html/images/phocafavicon; and then upload your new favicon (or even your logo) in that folder. Go back to your joomla admin backend, go to Components>Phoca Favicon>Create Favicon. There you will find the image (favicon) you have uploaded and do not forget to select the template to apply the new favicon to.
Maybe there are other simpler ways. Personnaly, I have used option 1 until I discovered 2.
Good luck
