Binding to textbox and updating source table

So, I have a form, containing several textboxes.

When a user click elsewhere on the form, the texboxes are bound/populated with values from a single record.
(Parent-child)

However, where I am struggling, is that I need any  manual edit of those textbox values to be written back to the originating table record.

Despite lots of googing - i'm still trying to get my head around how to get my textbox edits to write back.
Hope someone can point me in the right direction ?

Thanks

public void bindCurrent(string attachmentIdx)        {           
 // sql used to select data           
  string sql = "SELECT * FROM Ticket_Attachment";          
 // get an open connection to DB           
  sqlexp ce = new sqlexp();           
  SqlCeConnection cnn = ce.openCnn();           
 // data apapter > fill           
  SqlCeDataAdapter da = new SqlCeDataAdapter(sql, cnn);     
  SqlCeCommandBuilder cb = new SqlCeCommandBuilder(da);     
  DataTable dt = new DataTable();           
  da.Fill(dt);                    
  // bs is BindingSource   
  bs.DataSource = dt;           
  bs.Filter = "idx = '" + attachmentIdx + "'"; // idx is primary unique not null          

 if (txt_at_idx.DataBindings.Count == 0){
    txt_at_idx.DataBindings.Add("Text", bs, "idx"); 
 }       
}