"For ten of the same letter you receive the [Event] Veteran Fire Demon’s Tome." But how?

So, if I understood correctly, I can exchange ten of the same letter for one Demon's Tome. But how do i do it?

I cannot find anything related on any shops, like an item or something like that.

Can you please help me?

Thank you

Quote from LevinSnakesRise

Edited as I misread this originally;

You should be able to just combine the 10 of the same letter to get one, if I recall.

"You can't use the letter".
This is what i get if I try to just right click them..