All the screws and fasteners holding the window in the opening are accessed from the inside.  Look for screws (or nails in frame homes) at the top, sides, and bottom.  Remove them all.  Take another look just to be sure as some of them can be sneaky.
 

Note the head of a screw in top right corner.
 

Right in the middle of this picture is another screw which was in the side of the window.

After removing all the screws I went outside and cut loose all the caulking that had been applied over the years in an attempt to seal the window.  A couple good pushes on the inside of the window and the old window was out.  I used a shop brush to sweep all the debris and cleaned up the opening a bit.  I applied caulk all around the opening as per the window instructions and slipped the replacement in place.  Once sure it was firmly in the opening, I went back inside and screwed it in place.  In my case the bucks, or wood stripping around the window opening, was in good shape so I reused it.  IF it had been bad, I would have replaced it with pressure treated wood.  Keep in mind, these windows are in a cement block house, not a frame one.

With the inisde screws in place, back outside to final caulk the window and step back to admire your accomplishments.  It is a worthwhile project and if you are reasonably handy, YOU can do it too!