By the book this needs to be done by a registered engineer as its a pressurised system. That said, its a really easy job:
Find the correct PRV (mine was from an Ebay company but an OEM spec part - don't go cheap carp as its a safety critical item).
Depressurise the system and drain down...
I have a Complheat 80 boiler which vents horizontally through a wall into the alley between the house and garage. The alley is covered but open at both ends - and has been like this for at least 10 years before I moved in. The boiler plume is annoying, so can I install a plume kit to send it...