Remove the interior and exterior face trim from the window using a crowbar.
Insert the blade of the reciprocating saw between the exterior of the window frame and the wall opening. Cut through anything holding the window to the wall. There are usually several nails at the top and bottom of each side of a window, and sometimes nails also extend out the top and bottom of the window frame.
Grab hold of the window frame and attempt to move it after cutting all the way around it with the reciprocating saw. If it won't move, look around its edges with a flashlight and try to find nails or connectors you missed. If you find these, cut them.
Pull the window out of the rough opening with your hands. Use a crowbar to help it along if necessary.