I’d like to update my theme X to 4.6.2, but it says somewhere that “all customizations done to theme files be lost.” What exactly does this mean? Should i perhaps not do this update?
Have you ever modified X core files? If so, please move modified files to child theme to make sure it won’t be overwritten during update. If you’ve never modified core files you shouldn’t be worry about this.
However I advice you to backup your site before updating.