Well, you have two options to replace it. You can replace the board and reassemble the floor. But, this process isn't always practical. Also, you can cut out the damaged board and glue in a replacement.Definitely, you should hire efficient carpenter for that work, as it is very difficult process to replacing a board on a hardwood floor.
Also, you can check at http://www.hotfrog.co.uk/find/wood-flooring as it may helps you more.