In the theme’s files, please access /rey/inc/files/ and you can find the rey-child.zip archive which contains the child theme.
Simply install it like you install any theme in Appearance > Themes > Add new and activate it.
When you’re changing the active theme from parent to child, or child to parent, it’s likely the outcome to be disastrous. That’s because the Customizer settings were previously saved onto the active theme. So the solution is to just migrate the settings.
To migrate the settings, access Customizer > Import Export, and export the active theme’s settings. Then, go into Themes, change to parent or child and access again Customizer > Import Export, and import the previous settings.