Wooden floorboards damaged by water or trauma present a problem: The boards connect to the neighbouring boards by tongue-and-groove milling, and it will damage those adjacent boards if you try to pull the damaged one straight out. Getting the board out takes some finesse, and the new one has to be altered to get it back in. Make sure you have the same size and style boards to install as replacements.
Set one of your replacement planks on a work surface. Set your circular saw on top of it, with the blade sitting off the edge of the board. Loosen the adjustment nut on the side of the saw and adjust the blade depth to match the thickness of the board, which will generally be between one-half inch and three-quarters inch.
Set the circular saw over one the damaged planks, with the saw facing in the direction of the plank's length and the front edge of the blade guard pressed to the surface. Engage the saw and bring the blade down into the wood. Run the saw forward over the length of the board, without letting the blade go within an inch of the edges of the board.
Repeat the process to make several more cuts up the middle of the board, letting the cuts cross over each other, so the wood starts coming out in pieces. Cut as out much of the middle of the board as you can without touching the edges.
Knock the edges of the board inward with a hammer and chisel, pushing the edges of the board off the edges of the adjacent boards. Get the entire board out.
Set the replacement board facedown on your work area. Use your utility knife to cut off the bottom lip from the groove that runs along one edge of the board.
Press the replacement board into the space, setting the tongue side of the board first into the groove of the neighbouring board, then dropping the other side of the board, with the altered groove, over the tongue of the board on the other side. Shoot pairs of nails every foot to set it in place. Repeat for each board that has to be replaced.