By default, Gmail shows the most recent message in a conversation at the bottom of the thread. If you want to see new messages at the top of the thread, you can install an extension from the Chrome Web Store.
To add the extension to Chrome:
Open the Chrome Web Store.
Click in the Search box and type Reverse conversation for Gmail
Click on the Reverse conversation for Gmail extension
Click on Add to Chrome
Click on Add extension in the pop-up window for confirmation
Once you restart Gmail, you conversations will show in reverse order, with the newest message at the top of the thread. You will see buttons to reply to or forward the latest message along the top of the message, as well as any suggested responses.