There are alternative repositories from which you can install later versions of tmux, but they are not official. Installing from source is relatively easy:
# install any dependency packages needed for building
sudo apt-get install -y exuberant-ctags cmake libevent-dev libncurses5-dev
# download link from official http://tmux.github.io/ site
wget https://github.com/tmux/tmux/releases/download/2.0/tmux-2.0.tar.gz
tar xvf tmux-2.0.tar.gz
cd tmux-2.0
./configure && make
and if that all worked, make sure all running tmux sessions are stopped (because the protocol version might differ)
sudo apt-get remove -y tmux
sudo make install