I chose 'fill form' under 'preview form'. How to clear this?

I deleted all my browsing data and it still is not cleared.

Please note that if you have enabled "Auto Fill" setting in form, it will be cleared only when the form is submitted or browser cookies are cleared. 

Please check the screenshot below on how to disable auto fill setting of form:  

Alternatively, you can clear browser cookies that will delete saved form field data. You may like to take a look at the following guide on how to clear browser cookies:   https://www.wikihow.com/Clear-Your-Browser%27s-Cookies

Many thanks!
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