If your theme is changing while you update it, then it means that somebody has not used the child theme, but changed the theme files of the parent theme. In this case we cannot do anything, you would have to copy all of the changes from the parent theme to the child theme.
The parent theme always updates all the theme files so you cannot change theme, this is what child themes are made for.
You should insert custom CSS and JS into theme options>custom css&js if you do not want to use a child theme. Those settings are stored in the database and will not get changed while you update the theme.
You won't lose the Content or Theme Options - they are stored in the database, so you would have to corrupt or drop your database first to lose your settings or content.
Bryan told you only about theme files customizations, they will be lost when you didn't apply them on the Child Theme.
So, to prevent losing customizations of the theme files, you have to apply them to the Child Theme, how to do it? All about it here:
I have created a child theme and was trying to update the betheme without changing the custom design. I getting them error and not able to update my betheme. Please help me out.
The update process is starting. This process may take a while on some hosts, so please be patient.
An error occurred while updating Betheme: The package could not be installed. PCLZIP_ERR_BAD_FORMAT (-10) : Unable to find End of Central Dir Record signature
I'm trying to solve your problem, please stay calm.
Please disable all of the plugins, clear the cache, switch the theme to parent and check if the issue still exists.
This problem could be solved that way, but usually it's problem related to the hosting provider, not the theme. So if it won't gonna help you, we ask you, to contact the hosting provider and ask him about some logs, he should recognize the problem after looking at the error message which will be in the logs.
Comments
https://wordpress.org/plugins/duplicator/
thanks