First, make sure that your website template is connected to a valid (not expired) e-commerce plan - you should see a banner under the template thumbnail.
Access the store settings from the dashboard
Go to My websites → Hover the mouse over the three dots on the template thumbnail → Store manager.
Or, go to Settings → Online store → Open store manager.
Access the store settings from the editor
Within the editor, find the store page/section, and click on Manage store.
Or, click on the Online store button on the left toolbar → Open store manager.
Or, you access the store manager via website settings from the left toolbar.