how to use the env variable to change the value of it in config.dev and config.prod in cfg.json in AEM as a cloud service

Please check https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/implementing/deploying/configuring-osgi.html?lang=en#types-of-osgi-configuration-values

Environment-specific values, which are values that vary between Development environments, and thus cannot be accurately targeted by run mode (since there is a single dev runmode in Adobe Experience Manager as a Cloud Service). For example:

{
 "url": "$[env:server-url]"
}

Note that a single OSGi configuration file can use any combination of these configuration value types in conjunction. For example:

{
"connection.timeout": 1000,
"api-key": "$[secret:server-api-key]",
"url": "$[env:server-url]"
}