How to dynamically change usercontrols? asp.net, c#, code-behind

I need to learn how to switch out usercontrols programatically.
I have two controls: ppLoggedOutMenu.ascx and ppLoggedInMenu.ascx.

I want to put the code in the page load method. I already have some code that is checking whether a cookie exists so I was hoping to just drop some code in to add the ppLoggedInMenu to the "(If cookie != "" )," portion of the code, but I can't seem to figure it out or find an example of how to do this.

Can anyone help?

Thanks,
MHenry