Replacing a broken window spring is easy for the average do-it-yourselfer to do, and the task doesn't require any special tools. The only difficulty in replacing a broken window spring occurs if the window is so large that it requires two people to hold it up.
To replace a broken window spring, remove the plastic stops inside the window frame. Pull out just the bottom of the take-out clips inside the frame without removing the clips completely. Lift the window sash out of its frame. Raise the spring balance carefully until you can unhook it from the window frame. Replace the broken spring, and return the window sash to its position in the window frame.