first test if its set:
echo $PYTHONPATH
if you want to set it temporarily
export PYTHONPATH=/home/:$PYTHONPATH
note that you add the desired path you want (in my case /home) and additionally append the $PYTHONPATH variable othewise you'll delete your exisiting path and will only get /home on your path. this is important!
if you want to make it persistent add the command on ~/.bashrc.do
vim ~/.bashrc
add the command at the end of the file
export PYTHONPATH=/home/:$PYTHONPATH
reboot and check if your desired PYTHONPATH is set doing:
echo $PYTHONPATH