How to purge the cache in phpBB3
You can clear the cache in phpBB3 from the administration control panel of your forum. After you log in to the admin panel the default page that is displayed is the General tab page. There is a section on that page called Resynchronise or reset statistics. In that section there is an option labeled Purge the cache. Click on the Run now button next to the Purge the cache option. After that you'll be asked to confirm the action, so just click on Yes.
Another thing you can do from the administration panel is to delete only the cached style templates. To do this click on the Styles tab of the admin panel, then click on the Templates link on the left side of the Styles page. On the Templates page that opens click on the Cache link for the particular style, then mark the checkboxes for the template files and click on the Delete marked button. This will only delete the cached template files and not the original template files, so you don't have to worry about deleting anything essential. Keep in mind that purging the cache from the General page of the admin control panel will also clear the cached template files.
Clearing the cache from the administration panel might leave some cached files in the cache folder inside the root phpBB3 directory on your hosting account. If your phpBB3 site is installed, for example, in a folder called phpbb3 in the root public_html directory on your HostKnox account, the path to the cache folder would be public_html/phpbb3/cache. You can delete all cached files manually using an FTP client or through the Files section of the Pixie control panel. However, DON'T delete the files index.htm and .htaccess.