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.
I also found another good article:
ClickOnce: Bringing Ease and Reliability to Smart Client Deployment