You'll need to run a few commands in terminal to update your OpenBazaar version from source.
#Updating the client
In the OpenBazaar-Client directory:
git pull
npm update electron
#Updating the server
In OpenBazaar-Server directory:
git pull
pip install -r requirements.txt
It's possible your python version is out of date. If you get an error running the server, you need to update your python version to 2.7.9 or newer. If you want to use an existing PPA, follow these instructions:
sudo apt-add-repository ppa:fkrull/deadsnakes-python2.7
sudo apt-get update
sudo apt-get install python2.7 python2.7-dev