after selecting Microsoft excel library in refrence 
section  we can create object for excel by CreateObject
("Excel.Application")

place two textbox and commandbuttons in design window
and copy this code in code window
-----------------------
Dim xl As New Excel.Application
Dim xlsheet As Excel.Worksheet
Dim xlwbook As New Excel.Workbook

Private Sub Command1_Click()
    Text1.Text = xlsheet.Cells(2, 1)
    Text2.Text = xlsheet.Cells(2, 2)
End Sub

Private Sub Command2_Click()
    xlsheet.Cells(21, 1) = Text1.Text
    xlsheet.Cells(21, 2) = Text2.Text
    xlwbook.Save
End Sub

Private Sub Form_Load()
    Set xlwbook = xl.Workbooks.Open("C:\Documents and 
Settings\bct20vb\My Documents\sa.xls")
    Set xlsheet = xlwbook.Sheets.Item(1)
End Sub

For Using Excel in Vb,we have to add reference of Microsoft 
Excel libery. After That we can create object for excel and 
we can able to use Excel in vb