How do I change an web.config setting using transformation syntax?

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I have a value stored in my web.config file that I would like to change when the site is published. I want to change it from TEST to LIVE.\[code\]<appSettings> <add key="RequestMode" value="http://stackoverflow.com/questions/15683854/TEST" /> // other keys here</appSettings>\[/code\]Is this possible using web.config transformation syntax? If so, how?Thanks.