Name changes are not allowed
Once a ticket is issued, the name on it stays. That is the airline’s rule rather than ours, and it is why checkout asks for names as printed on the ID.
So check every name against the passport or ID that traveller will fly on, before you pay. It is the one part of a booking that cannot be put right afterwards.
What checkout asks for
For each traveller: first and last name as printed on the ID, a date of birth, and a gender. The blocks are numbered within a type — Adult 1, Adult 2, Child 1 — so they match the party you searched for.
The email and phone number are for the booking as a whole rather than for one traveller. Both are shown back on the booking’s own page.
Why the date of birth has to match
It has to agree with the kind of traveller: an adult is 12 or over, a child is 2 to 11, and a lap infant is under 2. A date that disagrees is refused, because the age is what sets the fare — see flying with children.
If somebody was booked as the wrong type, that is a different fare and not an edit. Cancel it where the fare allows that, and book again.