Excel worksheet refreshed from textbox on button click Visual Basic - vb.net

Question

I want to make a code which will send a textbox value to an excel worksheet. Every time i press the button the text box value shoud replace the previous one. What i ve already done is just send this value to an excel worksheet, but in every new click there is a new Workbook opening. what should i change ?
here is the code
Imports System.Data.OleDb
Imports Excel = Microsoft.Office.Interop.Excel
Public Class Form1
Dim objApp As Excel.Application
Dim objBook As Excel._Workbook
Dim objBooks As Excel.Workbooks
Dim objSheets As Excel.Sheets
Dim objSheet As Excel._Worksheet
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
' Create a new instance of Excel and start a new workbook.
objApp = New Excel.Application()
objBooks = objApp.Workbooks
objBook = objBooks.Add
objSheets = objBook.Worksheets
objSheet = objSheets(1)
objSheet.Range("A1").Value = TextBox1.Text
'Return control of Excel to the user.
objApp.Visible = True
objApp.UserControl = True
'Clean up a little.
objSheet = Nothing
objSheets = Nothing
objBooks = Nothing
End Sub
End Class

Answer 1

Change this:
objApp = New Excel.Application()
In
If objApp Is Nothing Then
objApp = New Excel.Application()
End If

Answer 2

I'd simplify as follows:
Public Class Form1
Dim objApp As Excel.Application
Dim objCell As Excel.Range
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
If objCell Is Nothing Then
objApp = New Excel.Application()
objCell = objApp.Workbooks.Add.Worksheets(1).Range("A1")
End If
objCell.Value = TextBox1.Text
'Return control of Excel to the user.
objApp.Visible = True
objApp.UserControl = True
End Sub
End Class
this code should be made more robust to handle such situation as user closing Excel

Answer 3

this finally did the job
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
If objApp Is Nothing Then
objApp = New Excel.Application()
End If
If objBooks Is Nothing Then
objBooks = objApp.Workbooks
End If
If objBook Is Nothing Then
objBook = objBooks.Add
End If
If objSheets Is Nothing Then
objSheets = objBook.Worksheets
End If
If objSheet Is Nothing Then
objSheet = objSheets(1)
End If
objSheet.Range("A1").Value = TextBox1.Text
'Return control of Excel to the user.
objApp.Visible = True
objApp.UserControl = True
'Clean up a little.
objSheet = Nothing
objSheets = Nothing
objBooks = Nothing
End Sub
thank you all for your responses