“Rhuk_milkyway” is the default Joomla template, so if you haven’t changed your template, the file will be located in this folder. Open the CSS folder to display a number of files with the .css extension. The one you are looking for is template.css. To edit, open this file with your text editor.
Use control + F keys to open the find option in your text editor. Type in mw_joomla_logo.png and replace it with your own filename example: my_logo.jpg (You may also want to change the width and height of your image in the two lines above where mw_joomla_logo.png appears.)
Upload the css file onto your server in your /templates/rhuk_milkyway/css/ folder.
Almost there… One more step. Upload your logo image to the images folder which is also stored in your template folder. /templates/rhuk_milkyway/images/
Refresh the page and the new logo should now be active.