If you updated your .tmux.conf file and find that it is not working, perhaps you have existing tmux session running. You need to exit all tmux sessions so that a new tmux server will start and (re)read the updated .tmux.conf file.
If you do not want to stop all tmux sessions, you can also issue the source-file command either from within tmux or use tmux at the command line.
# see what other tmuxes are running
ps -ef | grep tmux
# if control-b is your tmux prefix key, type:
c-b : source-file ~/.tmux.conf
# or at the command line:
tmux source-file ~/.tmux.conf