The favicon is the small image that is displayed in the browser bar and tabs when you open a page. What is the use of the favicon? It helps to identify a certain website when having multiple open tabs in the web browser or accessing a website from the bookmarks.
Joomla has it own favicon and by default each new Joomla installation will have the same icon. If you are using a third party template, probably the designer has included its own favicon in the template and your site will display that image. Here I will show you how to change that small image from the browser tab:
1. Find or create the image you would like to use. Favicons are not very detailed images and usually are square or rounded.
2. Resize the image to 16×16 or 32×32 pixels and convert it to .ico file. With this tool http://tools.dynamicdrive.com/favicon/ your image will be automatically resized and converted to the required file format.
3. Save the files to your computer and make sure the file is called favicon.ico
4. Using FTP or a file manager tool, upload the image to the following folder of your website:
and overwrite the old file.
Please note that most of the browsers use caching to speed up loading time of the website, and some elements such as images and favicon are stored and loaded on your local computer. This means that your new favicon will not show right after you upload it, even refreshing the page couple of times. Probably you will need wait couple of minutes or delete browser cache and restart the browser to see the change.