Does anyone know how these functions work?

Question

  Change ()   Lowercaseletter ()   Capitalletter ()   Lowercaseorcapital ()

Answer 1

Those do not appear to be standard functions. Did you write them yourself, or do you want to implement such functions? In that case, we would need more information to be able to help you.
Just in case, this is what C++ provides regarding lowercase and uppercase letters:
https://en.cppreference.com/w/c/string/byte/tolower
https://en.cppreference.com/w/c/string/byte/toupper
Both functions can be applied over a string using the std::transform function:
https://en.cppreference.com/w/cpp/algorithm/transform

Answer 2

Shadow No, I didn't write these functions, thank you🙏