Use a ratchet to lift and remove the inner casing.
With the trim removed, along with any insulation molded into the gap between the window and the house frame, you’ll see the nails or screws that hold the window in place. Use a reciprocating saw or hacksaw with a metal cutting blade to cut all of these attachments.
Remove the window by having one person push from the inside, while one or two helpers on the outside catch the window as it comes free. Even small windows are heavy, be careful and be prepared. If the window frame height is low enough, put sawhorses or scaffolding to “catch” the window when it comes out.