If you’ve followed the installation steps in the User Guide, than you should now have a Theme Options item in your Admin Panel. There you can set a custom logo image. Here is how the controls for that look in the Admin Panel:
In case you don’t know where to find the User Guide:
Go to https://themeforest.net/downloads. Find the theme and click ‘Download’ > ‘All files & documentation’. Download and unpack the zip file. The User Guide will be in the ‘Documentation’ folder.