Changing the password for a drive can be accomplished in the Edit Drive dialog. From the Drives tab select a drive and press the Edit... button. In the Edit Drive dialog press the Change button next to the password field.
After providing the current and new passwords the change password operation will begin immediately. Changing the password requires all encrypted files to be updated on disk and may take some time depending on the number of files (the size of the encrypted files is not a factor).
It is important to let the change password operation finish. If the process is interrupted some files will be encrypted with the old password and some files will be encrypted with the new password and manual intervention will be required to recover and sort the file data.