Step 1: Click on Checkout, then click on Behaviour and Edit Behaviour.
Step 2: Click on Behavior tab. Enable the Order Summary Open by Default button, if you would like the order summary to be open by default at your checkout.
Step 3: Once you are done, don’t forget to click on Save changes
You can set up to have order summary open by default at your checkout by clicking on the Checkout tab in the menu of your Admin panel of the selected store.