If your requirement is to store only one late fee for each member then you can retrieve the
DataRow
for the member by the PrimaryKey value i.e. MemberID and set the late fee as below
Dim memberRow As DataRow = membersTable.Rows.Find(memberID)
If memberRow IsNot Nothing Then
memberRow.Item("LateFee")= lateFee;
End If
You may accept and vote the solution if your problem is solved, otherwise please post your queries.