activate the child theme after editing main theme

i need to activate the child theme to avoid deleting all editing i do in the main theme when upgrading
but i have a-problem that the old developer had wor on the main theme and have edit it (css and functions page )
if now i need to activate the child with out loosing the editing (to upgrade the theme)
what can i do with steps please

Hello,

You need to move all your code customizations made to the main theme to the child theme’s functions.php file.

And CSS under the customizer will not be lost when you activate the child theme.

Please take a complete backup of the website before updating.

Best Regards

[Author]

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