We recommend using a child theme to make changes to the theme files because all changes made to the theme files will be lost after it updates.
1) You must have the Mercury theme installed.
2) Please, download the Mercury_3.9.2_Full_Package.zip file from the Downloads section. https://themeforest.net/downloads
3) Then install the Mercury child theme. You can find it in the Mercury_3.9.2_Full_Package.zip file in the Child Theme Example folder.
4) Activate the child theme.
After activating the child theme, the Customizer settings in the parent theme will cease to work. You will have to do them again.
5) Using the FTP client, you can copy a file from the parent theme to the child theme and make changes.
The style.css and functions.php files don’t need to copy, as they already exist in the child theme. You can only add changes to them.