In Svelte, we do this with stores. A store is simply an object with a subscribe method that allows interested parties to be notified whenever the store value changes. In App.svelte, count is a store, and we're setting countValue in the count.subscribe callback.
Click the stores.js tab to see the definition of count. It's a writable store, which means it has set and update methods in addition to subscribe.
Now go to the Incrementer.svelte tab so that we can wire up the + button:
Clicking the + button should now update the count. Do the inverse for Decrementer.svelte.