Answer
1.) After you download theme package from Themeforest, Extract the package and go to folder 'THEMENAME-vx-xx'
2.) Go to the path, 'include/plugin/tgm-plugin-activation/plugins/layerslider.zip'
3.) Extract the file then copy the folder 'layerslider'
4.) Access to your site via FTP, to the path 'wp-content/plugins'
5.) Replace the existing folder in this path with the one you copied from 3.)