How To Manually Update WordPress Themes Elegantly

Sometimes your WordPress theme just doesn’t update properly using the standard update method for whatever reason. In these situations, you need to perform the updates manually to take advantage of the latest and greatest enhancements to your active WordPress theme. This is a guide on how to perform these updates manually in an elegant fashion. If performed properly, this update process should only leave your site down with a clean maintenance landing page for at most five minutes.
Create a maintenance landing page. If you don’t want to make one from scratch I recommend using this Simple Maintenance Page template as a starting point. Customize it with your logo, favicon, styles and email address and save it as “maintenance.php, then upload it to your web server’s public root folder. You can view my maintenance page here. Make sure to add the following PHP code to it in order to set a 503 Service Temporarily Unavailable status code, you can easily verify the status code with httpstatus:
Create and activate a child theme if you haven’t already. Ensure all of your website settings, styles, and any other parent theme overrides are located within your child theme
Source: https://managewp.org/articles/18260/how-to-manually-update-wordpress-themes-elegantly

source https://williechiu40.wordpress.com/2019/01/05/how-to-manually-update-wordpress-themes-elegantly/