If older decks of pressure-treated wood don't receive annual cleaning and sealing, they tend to deteriorate. The likely result you'll encounter will be deck boards that cup, splinter and disintegrate. Make repair of your deck boards a priority for safety and aesthetic reasons.
Extract the nails or screws from the damaged deck board. Lift it off the joists.
Chisel out any rotted parts of the support joist. Spray sealer on the joist. Spray a second coat after the first one dries.
Cut a new joist to the length of the old one and spray with sealer. Let dry. Nail the sister joist to the damaged joist every two feet. Toenail the sister joist to the ledger and rim joist.
Cut a replacement deck board from matching lumber. Provide a weathered appearance to the board by scrubbing with a mix of one gallon water and one cup baking soda. Rinse and wait for the board to dry. Seal the board or stain it to match the original boards.
Attach the replacement deck board to the joists with galvanised deck screws.
If you have a special situation, such as a replacement deck board shorter than the original, cut a replacement long enough to span three joists. Cut out the damaged section of the original board using a jigsaw on either side of the joists. Glue and screw pressure-treated cleats on the sides of the joists at either end of the repair and attach the replacement deck board to the cleats with deck screws.