In order to update the theme via FTP manually, you can follow the below instruction:
Step 1: Download the Installable theme from ThemeForest in the Download menu on ThemeForest. Then extract it to your computer.
Step 2: Access your server via FTP or SFTP (Filezilla is a free recommended program)
Step 3: If you translated the theme and the language files are in themes/Cenos/languages/, taking the Cenos theme for example, you need to backup then upload them to wp-content/languages/themes/. If not, after updating the theme, any customizations to the theme’s core files, such as PHP files, language files will be lost.
Step 4: Browse to wp-content/themes/
Step 5: Delete the current theme folder (folder “Cenos”) then upload the new one downloaded in step 1.