Gmail: Reverse Conversation Extension puts the most recent email in a conversation at the top. This will also move the reply button from the bottom of a conversation to the top. To reverse the flow of the conversation, you must add an extension to Chrome. (Note: this is different than the most recently received email being at the top of your inbox.)

To add the extension:

1. Open the webpage in the Chrome Web Store.
2. Select Add to Chrome. (You may have to grant permission to the extension.)
3. When you close and reopen your Gmail the order of the emails within a conversation will be reversed. You may have to refresh Gmail to see the change.