It is really simple.
If you already have a local git repository from the original installation:
cd billy
git pull # to upgrade the repo from the original repo@github
ansible-playbook site.yml -K -e "installation_mode=upgrade"
If you do not have a local repo:
git clone https://github.com/cactusesecurity/billy
cd billy
ansible-playbook site.yml -K -e "installation_mode=upgrade"