This article answers your question. I'm pointing you to the article instead of just posting an answer because everything in the article is worth knowing.
http://www.sayedhashimi.com/CategoryView,category,ClickOnce.aspx
The following is the relevant excerpt from the article:
Forcing ClickOnce Updates
One of the big selling points of ClickOnce is automatic updates. One of the common questions I get with regard to updates is "How can I force an update on the user?"
There are three things to know with respect to forcing updates on users:
1) If your application is an online application, your users will always run the latest version; online applications get downloaded everytime the application is accessed. Thus, with online applications, you get forced-updates by default.
2) If your application is an installed application, you can force updates by using the MinimumRequiredVersion attribute. If you publish your application using Visual Studio, you can set this property from the Updates Dialog.
3) The last thing to note is that if your application is an installed application (and you have not set the MinimumRequiredVersion attribute) ClickOnce will prompt the user with an "Update Available" dialog ONLY if the user launches the application from the Start Menu shortcut. That is, if an application is an installed application and the user launches the application from a URL, ClickOnce forces the update.
Edit - Added
I also found another good article:
http://www.code-magazine.com/article.aspx?quickid=0601041&page=3