From the events list, click on an event to bring up the stats page.
If you need to make any changes including inventory, prices, event details, etc, click on Edit, which will bring up the familiar event set up form.
If you prefer to manage inventory in Shopify, you may switch the Inventory management setting for each variant to Shopify. This will also cause the fulfillment service to be changed to manual.
If you try to edit the inventory from Shopify without changing the inventory management setting first - it will appear to work - but then the inventory will be reset back to what it was before within an hour. So to re-iterate, either update the inventory from the Event Ticketing app, or change the inventory management setting to Shopify, and then change inventory levels in Shopify.