risolto così:
codice:
        <form name="formUpdate" action="DoUpdate" method="post">
            <input type="text" name="txtId" readonly="readonly" value="<%=request.getParameter("id")%>" />

            <input type="text" name="txtEntrate" value="<%=request.getParameter("entrate")%>" />

            <input type="text" name="txtUscite" value="<%=request.getParameter("uscite")%>" />

            <input type="text" name="txtCausale" value="<%=request.getParameter("causale")%>"/>

            <input type="submit" name="Submit" value="Submit" />
        </form>
grazie a entrambi!!