Boiler Heat not reaching upstairs apartment

You may need to bleed the air out of the system.

Is she the only one with a thermostat or are there more. What kind of heat is it hot water or steam. Hot water heat radiators would need to be bled. Steam heat does not. Here are instructions for bleeding hot water radiators. Still would like to know if there are more than one thermostat. Would be able to help you out better.

Another question I have. Are there pumps at the boiler and how many.

OK! Baseboard heaters means its hot water heat. Boiler does not cap off at set temperature. Only one boiler for all units, right? How many pumps on the boiler?

It is a 2 person task. One person is in the basement adding water to the system and a second person goes to each radiator and bleeds off the air. We used to tap on the radiator to communicate when we were at the radiator and when we got water because the person in the basement has to open and close a valve to let water into the system and shut it of while the person upstairs moves to the next radiator. The radiator is bled using a special key (looks like a small skate key, if you are old enough to remember metal skates) that opens the small valve at the end. Once water comes out of the valve you shut it off with the key and tell the person in the basement that you got water, then they shut off the valve until you get to the next radiator. You only want to add enough water to the system to replace the air. It isn't hard to do and communicating via cell phones may be better than banging on the radiator, but it takes at least 2 people. I think if you go to YouTube there might be a video.

Yep, just checked it is on YouTube. (Isn't everything?)

Probably not one pump. With three units and one boiler I would assume three pumps. Each unit thermostat would turn on the pump for that unit. Will wait for more.