This is an accepted solution.
Elias here with Shopify Support. That's a great question!
Just to clarify, since the screenshot isn't showing the entire page, are you trying to change the message that appears above the cart notes?
If so, the reason I'm asking is because "Enter Your Personalised Message Here, Make Sure No Spelling Mistakes" doesn't appear to be the default message. It's possible that you may have made the changes long time ago or someone from your team might have made the changes before. Nonetheless, if you've already enabled the order notes/cart notes on your cart page, you can absolutely change the message using the Language editor.
Simply click 'Actions' on the theme you're using and find Edit languages. After that, you'll be taken to the Language editor. From there, click on the 'Cart' tab and look for the Note field.
Here's a screenshot of this from my test store:
I hope this helps! If you run into any roadblocks or if you've any other questions, don't hesitate to let me know!