Measure the space for your uPVC window before you order it. The height and width of the uPVC frame should be ½ inch less than the space in the wall. The ¼ inch space around all sides of the new frame is necessary to accommodate the expansion of uPVC windows in summer heat.
Cover the floor and ground outside of the window with plastic sheets.
Break the window by lightly tapping it with your hammer at the top from the inside of the house. Wear gloves to protect your hands. The shards should fall on the plastic for easy clean-up.
Remove the sealing around the frame with your box-cutting knife. Saw the frame vertically and horizontally and remove it. Chisel away any cement or other debris from around the opening.
Run a bead of caulk around the space that will hold the window.
Put the uPVC frame into the space and secure it with the fixing screws provided by the manufacturer. The size of the screws and their location on the frame will vary according to the size of the window. The screws are typically 4 inches long set 24 inches apart and 6 inches from the corners of the window.
Run a bead of caulk around the edges to further seal the window.