What is involved in bleeding air from a multi residential hot water baseboard heating system installed in 1965?
Bleeding a water based central heating system is a DIY job anyone can do.

Find the bleed valves and crack each one open a bit (only one at a time though you don't want flooding). You should be hearing a hiss as air is pushed out. At some point water will get pushed out, careful it can be hot if the system is on, which is your sign to shut the valve again. If the hissing stops (no air is getting pushed out any more) then that means the systems needs more water.

Air will tend to collect at high points in the line. If there is any thought put into the system those areas will have bleed valves. Your best bet is to bleed from all of them regularly (once a year is fine).