How to reorder TMUX sessions?

They're sorted by name (which defaults to a number) You can change the names so that they appear in the order that you prefer.

Outside of tmux:

tmux rename-session -t 0 zzz

inside tmux: C-b $

Or:

c-b : rename session -t current_name new_name

The default ordering seems to have changed recently from name to index. If you want to put it back to name, try this in your .tmux.conf

bind s choose-tree -sZ -O name

If you'd like to "manually" order your sessions, you can prefix the session name with a zero-width or invisible unicode character. For example:

$ tmux rename-session -t charlie $(echo -e "\u200B")charlie
$ tmux rename-session -t beta $(echo -e "\u200C")beta
$ tmux rename-session -t alpha $(echo -e "\u200D")alpha
$ tmux list-sessions
charlie: 1 windows (created Sun Nov  2 12:39:30 2014) [284x87] (attached)
beta: 1 windows (created Sun Nov  2 12:39:27 2014) [284x87]
alpha: 1 windows (created Sun Nov  2 12:39:23 2014) [284x87]

The drawback is that you still have to refer to the session using those characters.