'textbox1.text=0123456789
Dim mystr As String
mystr = TextBox1.Text.Substring(0, 3) 'mystr="012"
Dim mystr1 As String
mystr1 = TextBox1.Text.Substring(3) 'mystr1="3456789"
mystr = mystr & "2" 'add "2" to mystr
TextBox1.Text = mystr & mystr1 'textbox1.text显示 01223456789
'还有很多的其他方法,你仔细想一下就ok了,写程序就是要你自己多多的想一想啊