Write functions capitalize-a-string, upcase-a-string, downcase-a-string for these purposes.
As examples of their use, please study the following:
(capitalize-a-string "monkey") => "Monkey"
(upcase-a-string "monkey") => "MONKEY"
(downcase-a-string "MONkey") => "monkey"
Hint: I suggest that you program the necessary functions yourself. Convert the string to a list of ASCII codes, do the necessary transformations on this list, and convert the list of modified ASCII codes back to a string. The Scheme functions list->string and string->list are useful.
Hint: If you want to make life a little easier (and learn less from this exercise...) you can use the Scheme functions char-upcase and char-downcase, which work on characters. But these functions do maybe not work on the Danish letters, so you you probably need some patches.