I found the answer posted by hachre on another forum:
On an existing install run all of these, on a fresh install run all except the mv:
sudo -i
apt-get update
apt-get dist-upgrade
apt-get install git
cd /root
mv mailinabox mailinabox_old # On existing installs only
git clone https://github.com/mail-in-a-box/mailinabox.git
cd mailinabox
bash setup/start.sh
bash setup/start.sh # the first one might fail due to bug #1298, run it again to make it succeed