Unable to enter traveler's middle name in shopping cart. Can I go back and edit the name or should I continue with my order?

I'm ready to purchase my tickets but am concerned that since the original form I filled out to get my itinerary settled only had space for a first name, and that's what's showing up now in my shopping cart, what about the fact that my name should appear exactly as it is on my passport which in my case is first, middle and last name? Is there some way to go back and change that? Or will I be given the opportunity to change it when I check out? My itinerary expires 3/20 and today is 3/19 - I'd appreciate a very quick answer.

Thank you so much!
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