Manual:$wgCacheEpoch

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

For a quick copy-paste you can to use :.

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

Since version 1.17, you can use manual:$wgInvalidateCacheOnLocalSettingsChange to automatically invalidate cache whenever LocalSettings.php is modified. In earlier versions, code in LocalSettings.php did a similar thing.