Answer:
First-- Are you dealing with vinyl, wood, window or aluminum window base.
first you need to remove all of the trim & stopper moulding from around the inside of the window and look for the nails or screws that are holding the window trim-frame to the window box frame. Once all the moulding is removed take a piece of 1 x 3 wood which overextends the window frame by about 8 incheson either side and screw this wood slab to the windows 3/4 inch thick outer base wood framing -- this will keep the window from falling out onto the street as you cut the anchor nails or screws. Cut the nails with a reciprocating saw/ with metal saw blade
I actually don't know what type of window you have to work with .
This is answered easier with a photo of your particular situation-- a picture is worth a thousand words., and t pictures are worth more... of course!
There are How to Videos on the internet for the Do It Yourselfer. Good Luck.