I'm not familiar with piaware but a quick google suggests it's based on raspbian so you'd want to do something like this:
sudo dpkg -i kplex_1.4-1_armhf.deb
Then you'd want to configure it by editing /etc/kplex.conf but to help with that I'd need to know what you want to multiplex to what.
*After* it's configured you can make sure it'll start on boot:
sudo systemctl enable kplex
...and then start it up...
sudo systemctl start kplex