Flask incorporates your changes automatically without you having to do anything.
Note how even though you made a change to your code you didn’t need to stop and restart your server to see the change you made. This is another of the advantages of using a framework like Flask which includes a web server which does automatic code reloading.
So most of the time you don’t need to stop your server to see changes you make, but if you get stuck not seeing a change you made it is sometimes if rarely necessary to do so.