How to insert a content place holder in update panel?

No if you put ContentPlaceHolder in UpdatePanel it won't work, however if you want to use UpdatePanel then you can use it in Content page. Just wrap all the controls on Content page with UpdatePanel:-

<![CDATA[<%@ Page Language="C#" MasterPageFile="MasterPage.master" Title="UpdatePanel in Master Pages" %>
<asp:content id="Content1" contentplaceholderid="ContentPlaceHolder1" runat="Server">
    Content Page<br />
    <asp:updatepanel id="UpdatePanel1" runat="server">
        <contenttemplate>
        <fieldset>
        <legend>UpdatePanel</legend>
           <asp:calendar id="Calendar1" runat="server"></asp:calendar>
        </fieldset>
        </contenttemplate>
    </asp:updatepanel>
</asp:content>

For details please go through these sites:-
Using the ASP.NET UpdatePanel Control with Master Pages[^]
Masterpages and MS Ajax[^]

Hope it will you.
Best of luck.