I had a look at the site and the FAQs, etc. and couldn't find anything there that would let you do it with the exception of going in and changing your appointment date. That should give you a new letter with barcode. (Why didn't you print it out the first time?)
Having said that, as it needs to be done with more than 1 day before the scheduled appointment, you might want to email them and the address at the bottom of the page to see if they can help. Otherwise, try changing the time/date of the interview. Or, better yet, call ICA and eventually, they will be able to transfer you to somebody who can help you. Just don't get too frustrated with the phone number punching system. I usually end up wanting to punch somebody instead of the constant numbers directing you to nowhere only to be cut off 10 minutes later.
