StrLower converts a string to lowercase and StrUpper converts a string to uppercase.
[Unit Sysutils]

Example:

procedure TForm1.Button5Click(Sender: TObject);
const S1 : PChar = 'Lowercase and Uppercase';
begin
    Label1.Caption := string(StrLower(S1)) + ' && ' + string(StrUpper(S1));
end;

The text in Label1 will be:

"lowercase and uppercase & LOWERCASE AND UPPERCASE "