The passphrase of a container can be changed using the Change button. By doing so, one can sometimes even recover a container with minor damages. First, a file dialog will be presented by which the container must be chosen. Next you will be asked wether to use the backup block in order to repair the container if it is damaged. You normally say No to this question. But if this doesn't work and you assume that the container has been damaged you may try to say Yes.
After this has been answered you are queried for the old passphrase. When it has been entered correctly you can give a new passphrase which must be verified subsequently just as in the procedure of creating a container. If it could be verified a message box informs you that the passphrase has been changed successfully.
The backup block is a triply encrypted copy of the header block which is decrypted with the passphrase. If that block is corrupted by at least one bit which was flipped it wouldn't decrypt. But the same information can be gained by decrypting the backup block. If the backup block itself is not corrupted we exactly have the situation where we can recover from the damage.