1. make sure your new icon is in one of these formats: .gif, .png or .jpg format and it should be no more that 32 pixels X 32 pixels.
2.Now from your Drupal page go to Appearance -> Settings->Shortcut Icon Settings.
3.Uncheck the option "Use the Default Logo." Go to "Browse" beside "Upload Icon Image" and find your file
4.No go to "Save Configuration." And clear your browser’s cache.