On the first computer, backup the database using postgres dump tool:
pg_dump thingsboard --host=localhost --username postgres -W > thingsboard.dump
Then, backup the /etc/thingsboard/conf directory:
sudo tar cvfz tb.tar.gz /etc/thingsboard/conf/*
Copy thingsboard.dump and tb.tar.gz to the new computer. On the new computer, with a fresh install of Thingsboard as per the posted instructions:
Stop Thingsboard, then:
sudo su postgres
dropdb thingsboard # this removes the database
createdb -T template0 thingsboard
psql thingsboard -U postgres -W < thingsboard.dump
Copy the /etc config files:
cd /
sudo tar xvfz /home/ubuntu/tb.tar.gz
Start Thingsboard.