The portal allows you to easily scale up or scale down the App Service plan that you are using. It also allows you to automate the process to allow you to scale the service plan on demand.

First log in to the Azure portal and navigate to the app service that you want to scale. From the left side panel, scroll down to find the Scale up (App Service plan) link. When you click on it, a new screen will show up and will allow you to select the pricing tier for your application:

There are plenty of service plans available for you to select by choosing the correct pricing tier. The basic tiers will have less performance than the standard or the premium tier, but will allow you to save more money. The premium tier will charge you more money but will give you the best performance.

The other link, Scale out (App Service plan), will allow you to override the instance count. It also allows you to enable/disable the auto-scaling option. If you want to auto-scale the service based on some conditions/rules, click on Enable autoscale:

This will open another screen where you can create a condition and ask the service plan to scale based on it. You can select it either based on a metric or specific instance count. You can add more rules and conditions as per your needs. At the end, click on the Save button to save all your changes made on this screen.

When you don't want to run the auto-scaling option, come to this screen again and click on the button labelled Disable autoscale: