Removing the Old Window
Cut away any caulking on the window trim using a utility knife. Then pry the trim from the window with a pry bar. Remove all the trim from around the window to reveal the nail fin and fasteners.
Remove any caulk from around the edges of the window frame. Use your utility knife to cut this material away.
Unfasten the window from the buck. If the window frame is fastened with screws, use a cordless drill fitted with a screwdriver bit to remove them. If the window is nailed, use the nail removing end of your pry bar to lift the nails. You can use a flat-head screwdriver to lift and expose enough nail shaft to get your pry bar end underneath.
If the nails are really giving you problems, you can use a drill bit to drill into and remove the nail head. Then the window frame can be lifted off of the nail shafts. The leftover shafts can either be pulled with pliers, pounded flush with the buck surface or cut free with a hacksaw.
Pull the window frame free from the buck.
Installing the New Window
Inspect and clean the buck. Scrape away any residual sealant and pound or remove any leftover fasteners. If the buck has been badly damaged or shows signs of weather wear, it may need to be replaced. Otherwise, if the buck appears to be in good condition you can go ahead and install the new window within it.
Seal any gaps between the buck and the brick wall measuring larger than 1/16 of an inch with masonry sealant.
Use a brush to apply liquid flashing around the edges of the buck and the brick window opening.
Select the appropriately sized window. The window should measure about ½ inch to 5/8 inch smaller than the buck within the window opening.
Position the window inside the frame and use shims to hold it securely. A series of shims should be placed along both sides of the window. The spacing, size of shims and number of shims used will vary depending on the window type and size. Consult the manufacturer's instructions regarding the proper technique for shimming the window.
Check the level and plum of the window. Use you level to determine if the window is seated properly in the window opening.
Fasten the window to the buck by driving nails or screws through the slots in the window's nail fin. Consult the window manufacturer's instructions regarding the size and type of fastener to be used.
Apply trim around the window. Fasten the trim with nails driven into the buck.
Seal around the edges of the window trim with weather-resistant caulking. Seal both the inside and outside seams where the trim meets the brick and the window frame.