One method is to select the character for a new game, then use the Customise button. When you get to the bio, delete it using backspace and rewrite. Or you might find it easier to copy your new bio from a text document, then Ctrl-V to paste it.
If you don't want to start a new game, you have a choice of editors, Leto or Gffeditor, available on neverwintervault.org. The character .bic file can be found in your nwn/localvault folder. Leto presents all the information in a user-friendly format, so perhaps the simplest option for players. As a modder, I prefer Gffeditor because it lets me change any game file; you just click on the Description field in the .bic file to change the bio, so really it's just as easy.
