Measure the rough opening for your window. If you are replacing an old unit, keep it in place for now and measure it from side to side and top to bottom.
Purchase a unit window unit. You will want one that's either equal to your existing unit or 1/4 inch smaller than the rough opening both length and widthwise.
Remove any trim around your existing window using a pry bar, both on the inside of the home and out.
Unscrew the fasteners holding the unit in place, both along the top sill and the bottom. This should simply be a matter of using a screwdriver or drill to remove the fasteners that attach the window to the wall framing.
Lift the old unit out of the opening carefully. It is best to remove it from the outside of the home so you don't drag dust and debris into your house.
Slide the new window into the opening, bottom first. Make sure that it is evenly spaced along the two side jambs. If the opening is uneven, use wooden shims to level it out.
Screw the unit into place by placing screws in the top and bottom jambs. Use screws that are long enough to penetrate the structural framing surrounding the window.
Install any trim kits and caulk all joints to ensure the opening is waterproof.