Hi Jeffrey,
How were your customizations added? Did you alter theme files directly, or use a plugin or child theme to make customizations? As long as you didn’t alter any theme files directly your customization should remain.
As for the manual update, you can follow these instructions:
1. Download and unzip the latest version of the theme.
2. Next, you will need an FTP program such as Transmit or Filezilla, or the File Manager within cPanel (found in your hosting account).
3. Login to your server using the FTP software or File Manager.
4. Upload the theme in to the /wp-content/themes/ directory, replacing the existing theme folder.
Navigate to Appearance > Themes and activate the theme.
I hope this helps. Thanks!