Manual:$wgCacheEpoch

Details
Set this to current time to invalidate all prior cached pages. Affects both client- and server-side caching.

If you are using Linux you can get the current date by entering the following on the command-line: date +%Y%m%d%H%M%S

Another way to invalidate prior cached pages is to tether invalidation to the modify timestamp of your LocalSettings.php. To do so, include this line in your LocalSettings.php if it is not already there.