C++ – how to change caption of button present on MFC dialog box

If all you want to do is display some text in a CEdit control then why not use it's SetWindowText function?

The following replaces the contents using your SetSel/ReplaceSel method:

void CTextCtrlAddDlg::OnBnClickedButton1()
{
    int start = 0;
    int end = m_editControl.GetWindowTextLength();
    m_editControl.SetSel(start, end);
    m_editControl.ReplaceSel(L"Test");
}

...where m_editControl is the edit control. If you want to append the text at the end, simply set the selection to the end:

void CTextCtrlAddDlg::OnBnClickedButton1()
{
    int end = m_editControl.GetWindowTextLength();
    m_editControl.SetSel(end, end);
    m_editControl.ReplaceSel(L"Test");
}

I agree with Goz though; some example code frmo you would help us identify what isn't working for you.

When you instantiate AnotherClass, pass it the dialog class:

class AnotherClass
{
private:
    CDialog& dialog_;

public:
    AnotherClass(CDialog& dialog) : dialog_(dialog) { }

    void MyFunction();
};


void AnotherClass::MyFunction()
{
    CClass* temp = (CClass*)dialog_.GetDigItem(IDC_ID);
    temp->doSOmething();
}