Sofas too big for windows

My bottom line recommendation is to get a smaller sofa, especially if you are renting. But even if you own the apartment, the cost can be more than you bargained for and if the frame gets damaged you get into more expenses. Also, if you have to move out after a year, what will you do? On the other hand, a few years from now when you want a new sofa, will you remove the window again or cut up the sofa.

up the stairs?

Many windows here are designed to be completely removable, particularly those fitted in apartment buildings.
You have to find out whether that applies in your case and, if it does, whether the opening will then admit the new sofa. Assuming that it is possible, the people who will be delivering the sofa will be able to remove and replace the window; it was done on our most recent move and it is a simple matter so long as the delivery men know what they are doing.