When you want to manipulate text in M you will find all the functions that you need in the Text object. For a full list of all the available Text functions: Text functions – PowerQuery M | Microsoft Learn

let
    varFirstName        = "John", 
    varLastName         = "Doe", 
    varFullName         = varFirstName & " " & varLastName, // "John Doe"

    varAllUpper         = Text.Upper( "John"),              // "JOHN"
    varAllLower         = Text.Lower( "John"),              // "john"
    varFirstCharacter   = Text.Start("John", 1),            // J
    varLength           = Text.Length(varFirstName),        // 4
    varSubString        = Text.Range(varFullName, 1,3)      // "ohn"
in
    varSubString