请参考下面的代码。
    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        nList = ListBox1.SelectedIndex  '事先定义的一个全局变量
        If nList > -1 Then
            Me.TextBox1.Text = ListBox1.Items(nList).ToString
        End If
    End Sub

'当TextBox1的值修改后按回车键时，保存回ListBox1原位置
    Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        If Asc(e.KeyChar) = 13 Then
            ListBox1.Items(nList) = TextBox1.Text
        End If
    End Sub

回复7楼：
下面是一套完整的程序代码。在TextBox1中修改完后，按回车或按按钮，均可把数据保存在ListBox1的相应位置中。

Public Class Form1
    Dim nI As Integer = 0
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        '假设序号由3位字符组成，姓名由8位字符组成，学号由4位字符组成，中间各插入一个字符
        '以下代码是为了演示而添加的数据，实际应用应该填入表中的记录。
        ListBox1.Items.Add("001 AAAAAA   1234")
        ListBox1.Items.Add("002 BBBBBB   3256")
        ListBox1.Items.Add("003 CCCCCC   6542")
        ListBox1.Items.Add("004 DDDDDD   7894")
    End Sub

    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        nI = ListBox1.SelectedIndex
        If nI > -1 Then
            TextBox1.Text = Mid(ListBox1.Items(nI), 5, 8)
            TextBox1.MaxLength = 8
        End If
    End Sub

    Private Sub TextBox1_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles TextBox1.KeyPress
        If Asc(e.KeyChar) = 13 Then
            Dim cSTR1 As String = ListBox1.Items(nI)
            ListBox1.Items(nI) = Mid(cSTR1, 1, 4) & TextBox1.Text & Space(8 - TextBox1.Text.Length) & Mid(cSTR1, 13)
        End If
    End Sub

    Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim cSTR1 As String = ListBox1.Items(nI)
        ListBox1.Items(nI) = Mid(cSTR1, 1, 4) & TextBox1.Text & Space(8 - TextBox1.Text.Length) & Mid(cSTR1, 13)
    End Sub
End Class