TrueCrypt is a program that allows you to create and manage encrypted volumes. You set aside disk space to store encrypted files in; TrueCrypt allows you (and anyone with the password) to access the files stored there as though the encrypted space was another disk drive.

To change the password on a TrueCrypt volume, select the file or device that contains the volume. Click on Volume Tools and select "Change Volume Password."

You'll be asked to provide the old password and confirm the new one. Make sure none of the keyfiles boxes are checked!

Congratulations! You've successfully changed the password on your TrueCrypt volume. Don't lose it!