How do I change an ending after getting the S ending.

[NullPointer]

Once I unlock all endings and the S ending for a letter, I can no longer change the result of the letter. Even if I go back and replay to get a bad ending, as soon as I exit it would default back to the S ending.

How do I choose to use a different ending for a letter. If I can't, will this affect my achievements in any way?

For example, if there is an achievement that depends on getting X in letter 1 and A in letter 2. However, before this achievement I got S endings on both letters. Does this mean I will never be able to unlock this achievement on this playthrough?

[whisperofapathy]

There is a point in the game where that feature is unlocked, and after you reach that point there should be a brown symbol on the right side of the title when you click on a letter in the chart. That allows you to pick the ending you want.
Hope this helps!

[NullPointer]

Excellent. Thanks for your answer!

[Katze]

I can't find that brown symbol...! Can anyone give me a bit more details? I am stuck...

[WMY Studio]

Originally posted by Katze:

I can't find that brown symbol...! Can anyone give me a bit more details? I am stuck...

Did you get the best ending of Li Wen's "Three Years After School"? You will unlock the story after getting that ending and will see the brown symbol when you click a block of a letter.