I would like to make changes to the page that appears after I submit the form (see attached). What file do I need to update to change the wording? Thanks in advance for your help...

Click on "Setting" then "Confirm Options".
Then alter the built in "Confirmation Message"

Thank you for the feedback. I found the text under the "Customize Payment Confirmation Message" option. However, the area that I want to change looks like it is found in "[_cart_summary_]". Where can the layout of "[_cart_summary_]" be changed?

Pick each element out separately.
[number] etc.....