Some store admins rely on the communication with the customers on daily basis and contact us plays a great role in the daily communication.
Today we are going to discuss the process of changing the default contact us confirmation message in nopCommerce.

Go to: Administration section > Configuration > Languages

Select "English" (for your store language) - View string resources <-- Click here
(If you have a different language setup then select that language)
In the value column > click "filter" icon
Paste this: "Your enquiry has been successfully sent to the store owner"
Click "Filter"

In the search result, you will see the value for Resource Name: contactus.yourenquiryhasbeensent
Simply click "Edit" and update the message in "VALUE" according to your requirements.
Hope it helps!