Replacing one or more boards in a hardwood floor is more complicated than it might sound. You can’t simply pry up the damaged board and pop in a replacement board, because the boards connect via tongue and groove milling on the sides that will rip off if you pull up the boards. You must extract the damaged boards in a way that keeps the adjacent tongue and groove intact, and you must alter the new board to allow it to drop in from above. See your flooring dealer for replacement boards.
Lay masking tape around the perimeter of the area of flooring you want to replace, so you don't accidentally cut into the boards that you don't want to replace.
Set one of the replacement boards on the work surface. Set your circular saw on top of it. Set the depth of the saw blade to match the thickness of the boards (generally about ¾-inch).
Use your circular saw to cut a line up the middle of the length of one of the damaged boards. Don't let the blade stray outside the board.
Cut additional lines up the middle of the board, repeatedly. Angle the lines over one another, so pieces of the floor start coming out in chunks. Get as much of the middle of the floorboard out as you can.
Place a chisel along one edge of the damaged board. Hit the chisel with your hammer to knock the edges of the board inward, off the tongue and groove fittings of the adjacent boards and into the area you cut out. Continue until the whole board is out.
Lay the replacement board on your work surface face down. Use a utility knife to cut off the lower lip of the grooved side.
Put the replacement board in place where the damaged board was, setting the tongue side in first and inserting it into the groove of the adjacent board, then dropping the rest of it down. The altered groove on the replacement board will allow it to go around the tongue of the neighboring board.
Secure the board with a finishing nail, shooting nails through the face every 8 or 10 inches along both long sides. Repeat for each board that you wish to replace.