It happened to me as well. Mine's was not a major mistake though, it was more like a typo - I had misstated my father's hometown. I wrote down the "correct" information on the form - I kinda remember reading it either in the guidance or answers to FAQ's, but definitely somewhere on the visa4uk website - and put my initials next to it. I didn't face a problem with UKNY about it at all.
Hope yours is not a major mistake as well. I think how serious the mistake matters a lot.
Good luck:)