set primary key to existing dataset

Hi.

I have a database in access and some new data in excel.
I need to compare the two databases, updating the access with the new items and updating existing ones (and leaving others as they are because they do not appear in the excel file)
I populated a dataset from an xls file and now I need to set the primary key.

How do I do that?

Thanks

Code:

Dim ficheiroxls As String

        ficheiroxls = txtArquivos.Text

        Dim connectionxls As OleDbConnection

        Dim connectionxlsstring As String

        Dim sqlxls As String

        connectionxlsstring = "Provider=Microsoft.JET.OLEDB.4.0;Data Source=" & ficheiroxls & "; Extended Properties=""Excel 8.0;HDR=YES"";" 

        sqlxls = "SELECT * FROM [Medicamentos(1)$]"

        connectionxls = New OleDbConnection(connectionxlsstring)

        Dim adapterxls As New OleDbDataAdapter(sqlxls, connectionxls)

        adapterxls.Fill(medicamentosxls)