iphone app: how to renew an expired distribution certificate through the iOS provisioning portal

Once expired, you will get a renew button, yes. If you want to do it before it expires, revoke the current certificate and you will get a button to request a new one.

Which ever way you do it, your ad hoc app will stop working tomorrow and you should distribute a new build with new provisioning profile signed with new certificate.

From the apple developer support center certificates section:

iOS Distribution Certificate (In-house, Internal Use Apps)

Users will no longer be able to run apps that have been signed with this certificate. You must distribute a new version of your app that is signed with a new certificate.