tmux is a terminal multiplexer, just like screen(1). However, tmux is actively maintained and is in my opinion much better. Due to these developments, it could happen that something stops working or becomes broken from one version to another. The upgrade from version 1.2 to 1.3 caused that the commands down-pane and up-pane were no longer working. My shortcuts were bound to these commands, so I got some error messages when starting the new version. Instead, you should use select-window instead with the target pane parameter:
down-pane becomes select-pane -D
up-pane becomes select-pane -U
While the Changelog suggests to use the flags -t:- and -t:+ respectively, it is a bit problematic since it does not rotate when you want to go from the last pane back to the first pane. So that's why I use the alternative flags. This might cause trouble if you prefer vertical splits, in that case you should use -L and -R. Not a perfect solution, but it works.