How can I unstick a window that been painted shut for years

You will need to go around the whole window with a putty knife to break the seal of the paint, after gently tap with a hammer to loosen any others. This should allow you to then open the window.

Tammy's is right - is should loosen but will take time and muscle!

If you have something flat-ish, like a dry wall mud blade or anything like that, and a hammer; put the blade where the window and frame meet, tap on the blade handle with the hammer gently. You have to do this all the way around, "breaking" the paint seal.

A razor blade may help in tight spots.

There's a tool for that! Actually several:

1. use a putty knife to open up the seams

2. use a utility knife or window opener-saw to open up the seams

3. use a handy bar or crowbar to help leverage open the window

Here's Tom Silva saving the day: https://www.thisoldhouse.com/how-to/how-to-open-painted-shut-window

You need to get a thin blade putty knife and hammer/tap it into the space between the sash and the casing, the sash and the sill, and between the top and bottom sash. In short you have to break the seal all the way around the bottom sash. Tread carefully between the top and bottom sash as there is often an interlocking weather strip. Once you get the sash free you could rub a little candle wax on to the side tracks. If you can reach the window from the outside (1st floor) you may have to do the same to the outside paint.

i agree with Tammy. If they are still to tight you can take a hack saw blade on edge to cut a 1/16th of an inch between the window and frame

same as above but if the window is still tight .......... take a hack saw blade on edge and cut a 1/16th" between the window and the frame