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Unfortunately, I wasn't able to replicate the issue. I cloned your form and I was able to change the text without any problems. Please check the screen capture below:
As you can see above, I selected the field first and then click the "Edit HTML" button. After that, the editor will appear and I can edit the text. After editing it, I clicked the "Complete" button.
If you're still having issues, please try using another browser to check if it will work or not. I will clear your form cache so that your forms will be refreshed from our servers.
Hope that helps. Thank you.