Functions.php file gets overriden by every update

If your theme supports child theme, then you need to create one and place that code in child theme’s functions.php file. This way, when you update the original parent theme, it won’t overwrite/wipe out your code. Please see here for more details on how to create child theme.

If your current theme supports child theme, then you shall create one and place your custom code in that child theme’s functions.php file. Please see here for more details on how to create child theme. Once you start using child theme, then updating original parent theme wont overwrite/wipe out your code.