Yes, update it on both (assuming all people involved are covered on the same 1095-A).
No, there is nothing else to state. Just use the correct/modified numbers for column B.
In most cases it is has the best overall outcome to allocate 100% to one tax return (and in most of those cases, 100% to the kid is best). Look at Line 5 of Form 8962 on both tax returns. Whichever one is lowest, allocate 100% to that tax return.
