You should use a build-release system like VSTS. Try creating an ARM template of your app and deploy this pipeline with VSTS. You can add value to the template.json file like here:
"appSettings": [
    // other entries
    {
        "name": "MyValue",
        "value": "[parameters('myValue')]"
    }
You'll require parameters.json which will hold the values:
{
    "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentParameters.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "name": {},
        "storageName": {},
        "location": {},
        "subscriptionId": {}
    }
}
And change the values of these parameters in VSTS portal.

This way you'll be able to implement a CI/CD pipeline used by professional sys admins.