Generally you can change the logo / upload your logo in the theme options (WP admin -> Appearance -> Theme Options).
If after changing the logo it doesnt change, mostly there is a problem with the media pop up. Unfortunately some plugins change the default media settings, and this cause the problem, the media pop up doesnt return the logo url. In this case it is important that the “Url file” Button is clicked / checked, then the media pop up will return the URL of the logo (see screenshot).
The button for the “URL file” you can find in the media pop up, after clicking the “Upload Logo” button.