I have a two storied home. When I turn on the heat the first floor becomes hot and remains warm when the thermostat is switched off. The top floor however is warm when the thermostat is on but becomes cold quite quickly after the heat is turned off. I've checked the windows even sealing all seams with tape, the a/cs are covered from outside and inside, and still the problem.
In the summer months upstairs is very hot while the 1 st. floor is comfortable.

Have you checked to see if the insulation in the attic is of the proper R- value? Depending of the part of the country you live in, it should be at least R-19, preferablly R-30.

Have you checked to see if the insulation in the attic is of the proper R- value? Depending of the part of the country you live in, it should be at least R-19, preferablly R-30.

I'm in NY. I kind of suspected it might be the insulation which probably wasn't changed or maintained since the house was built maybe 60 yrs ago.

Also, you didn't say whether you have one system or one for each level? If you only have one that is a problem itself. Different levels have different heat loads in summer and in the winter you will have heat rising.

One thermometer controls both floors.

Is there any way to adjust this? Perhaps through the boiler mechanism or the thermostat?

I have a two storied home. When I turn on the heat the first floor becomes hot and remains warm when the thermostat is switched off. The top floor however is warm when the thermostat is on but becomes cold quite quickly after the heat is turned off. I've checked the windows even sealing all seams with tape, the a/cs are covered from outside and inside, and still the problem.
In the summer months upstairs is very hot while the 1 st. floor is comfortable.
I'm still not clear on whether you have a system for each floor, I need more info, it could be more than one thing causing your problem.

There is one boiler servicing the entire building. Gas powered steam.

Probably the best way to figure out what to do is contact an expert, ask if they give free estimates? I am in the A/C bus. In Florida, but all problems are similar. I think for a home that has one system an multiple levels, zoning control makes the most sense. That will give you control over each floor via a thermostat and zoning control board.