How to change the value of attribute in appSettings section with Web.config transformation

You want something like:

<appSettings>
  <add key="developmentModeUserId" xdt:Transform="Remove" xdt:Locator="Match(key)"/>
  <add key="developmentMode" value="false" xdt:Transform="SetAttributes"
          xdt:Locator="Match(key)"/>
</appSettings>

See Also: Web.config Transformation Syntax for Web Application Project Deployment