There are two ways to open an existing board:

From the Home Screen

On the  home screen, locate the board in the Recent Boards list, or click All boards to search for it.

Opening a board from the home screen

From a Board

If you're already working from a board, open the Board Selection menu and select the board you need. If you don't see it in the list of recent boards, begin typing its name into the search box.

Opening a board from the board selection menu