When you are building a bot and creating new messages you can switch from Editing the messages to Navigating the messages by clicking on 'Switch to Navigation'.
You must have linked the message to the following one to be able to Navigate the bot.
The button 'Tea please' below already has the following message attached to it.
Click 'Switch to Navigation'
Now you are in Navigation mode.
Click 'Tea please'
The next message will open
Click 'Milk and Sugar'.
Now you see the 'Milk and Sugar' message that was connected behind the Quick Reply
This message is incomplete, and you are unable to edit in Navigation mode.
'Switch to Edit', the message parts will appear and you can continue building the bot.