1.) Access your website through your hosting service provider or through your Cpanel login. Once you are in the main section of the hosting area for your specific website find the website File Manager.
2.) Navigate to templates/(default template)/images. If you are unsure of which template is the default template, open the back-end of Joomla! and go to extensions/template manager. The template that has a star is the default template.
3.) Locate the logo.png file in the file manager. Some templates have multiple color variations and the proper logo must be found depending on which folder it is located in. It is wise to add the logo to all of the different color variations so people can change colors without needing to update the logo. You can learn more about which style variation or color variation you are using by looking in the Template Manger on the back-end of Joomla. Once you have located this file, upload your new logo, titled logo.png into this folder, and override the old logo.
4.) Check and see if the logo has been updated correctly. If the dimensions are incorrect, you will need to go into the template.css file to change the specifications. This file is in the Template Manager, click on the default template, and click on CSS editor. Here you can choose the template.css file. Edit this using the html editor and press control-F to search the page. Search for the word “logo” and you will find the place to change the dimensions. Usually you can choose the correct height and a larger % stretch for the width.