@Fang, Wenfei I think this issue is related to one of the threads where a similar case was reported. The user in this case also tried to update the file through online editor of the app service and tried to rebuild the app but the default answer did not get updated with the bot. The solution in this case was to use the VS editor to build and redeploy your bot to azure.
You should be able to download the bot source code from Azure portal by clicking on overview tab of your bot to see the option to download the code and run the solution on VS. Detailed steps are mentioned in the referenced thread.
Could you please check and try the same?
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