estou supondo que codigo seja numérico, e evento seja texto
Inserir:
INSERT INTO Agenda ( codigo, data, hora, evento )
SELECT " & txtcodigo & " AS Expr1, " & CDate(txtdata) & " AS Expr2, " & Hour(txthora) & " AS Expr3, '" & txtevento & "' AS Expr4;
Alterar
UPDATE Agenda SET Agenda.codigo = " & txtcodigo & ", Agenda.data = " & CDate(txtdata) & ", Agenda.hora = " & Hour(txthora) & ", Agenda.evento = '" & txtevento & "';
Excluir:
DELETE Agenda.codigo
FROM Agenda
WHERE (((Agenda.codigo)=" & CLng(gridagenda.Text) & "));