Unzip the file that you download from ThemeForest. In the folder that is created look for the file named theme-salutation-v1.5.1.zip and unzip it. Use an FTP application to upload the contents of the parallelus-salutation folder to the wp-content/themes/parallelus-salutation/ folder on your web server, overwriting the folders and files of the same names.
Note that there is likely to be a 1.5.2 theme update uploaded to ThemeForest later today.