You can change the heading of the collapsible order summary section on mobile from the Customizer.
Follow these simple steps to do that:
Step 1: Go to the ‘Checkout Form’ widget in Customizer
Under ‘Checkout Form’, click on ‘Form Style’.
Step 2: Scroll down to the ‘Mini Cart on Mobile View’ option
Here you’ll get the option to set the text when the order summary is collapsed/opened.
Change the text from here.
Step 3: Hit the ‘Publish’ button
There you go! You’ve now successfully modified the collapsible order summary text for mobile.