Application Insights is a Service on Microsoft Azure that lets you understand what users are actually doing on your App.
It also lets you diagnose any issues with it’s Powerful analytics tools and works with platforms including .Net, Java and Node.js.

The App Insights Instrumentation key is what is required to link your App with the resource on Azure.
If you already have an existing App Insights resource created through Visual Studio and you need to change it, then you can create another resource manually from the Azure Portal.

Once the App Insights resource is created, copy the Instrumentation key and replace it in your ApplicationInsights.config file. This lets you switch the ApplicationInsights resource for your Application.

Look for the InstrumentationKey tag in your ApplicationInsights.config file and replace. You might also need to change the InstrumentationKey in the HomePage JavaScript under Views folder added by App Insights SDK.

Start debugging your App and verify with your Live Metrics Stream in the App Insights resource that it is working.