Most often, the issues you have related to the update process are due to one of the following reasons:
Incompatible WordPress version
You can't run the latest WordPress version with an old theme or plugin version. Make sure the plugins and the theme is up to date. You can check the Changelog - https://vamtam.com/changelog/ if you doubt which one is the latest version.
If you are running plugins that do not come and are not supported by the theme, make sure they are compatible with the WordPress version you are about to update/install. If the updates don't fix the issue you encounter, then deactivate the plugins that don't come with the theme and test.
Old plugin version
Make sure that all plugins you are updating are compatible with the theme version and the WordPress version.
Complete points 2,4,5 of the Update guide.
CSS styles are not regenerated
Click on the "Save Changes" button in the theme options panel to re-generate the CSS.
Using FTP to update the theme
The bundled plugins cannot be updated using FTP. If you've chosen this, you have to switch to automatic updates to update the plugins bundled with the theme—complete points 2,4,5 of the Update Guide.
Renamed theme folder
If you change the theme folder name or the content of the style.ccs file, the automatic update feature will stop functioning.
If you are still in trouble after reading the guides, please read this guide - How to get theme support.