There is no "all" -- since that could mean tens of thousands of messages for some accounts.
Some options:
1 - Please long press on the account on the app's main screen -> options and folders -> account options -> adjust "messages to sync".
OR
2 - Scroll a message list down and it'll load more.
OR
3 - Use search, when initiated from a folder, it will search and bring up old messages.
Or all three as needed I suppose
