In continuation from here

We already covered here how to create the form, set references and place the required buttons.

Here is an example for merging cells A1 to E1.

Imports Excel = Microsoft.Office.Interop.Excel

Public Class Form1
    '~~> Define your Excel Objects 
    Dim xlApp As New Excel.Application
    Dim xlWorkBook, xlWorkBook2 As Excel.Workbook
    Dim xlWorkSheet, xlWsheet2 As Excel.Worksheet
    Dim xlSourceRange, xlDestRange As Excel.Range

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        '~~> Opens Source Workbook. Change path and filename as applicable
        xlWorkBook = xlApp.Workbooks.Open("C:\Tutorial\Sample.xlsx")

        '~~> Display Excel 
        xlApp.Visible = True

        '~~> Set the source worksheet
        xlWorkSheet = xlWorkBook.Sheets("Sheet1")

        '~~> Set the source range 
        xlSourceRange = xlWorkSheet.Range("A1", "E1")

        '~~> Merge the range
        xlSourceRange.Merge()

        '~~> If you want center alignment of the text horizontly and vertically then use this
        xlSourceRange.HorizontalAlignment = Excel.Constants.xlCenter
        xlSourceRange.VerticalAlignment = Excel.Constants.xlCenter
    End Sub
End Class