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You need to activate the parent theme to do the the changes.
All the main files are included in the parent theme.
A WordPress child theme takes the functionality of another theme, named parent theme. It gives you the possibility to customize an existing theme. Instead of modifying the theme files directly, you can activate the child theme and work within it.
All customizations are stored in the child theme and anything in the child takes priority over the parent. So if the same file exist in both child and parent, the file from the child will control that aspect of the theme.
If you need some other help please let us know regarding the theme.
Thank You.