There is no need to retain the installers. All updates are cumulative. The location of the download cache can be changed in the C3 settings page path tab. The settings page is accessed by clicking the gear in the upper right on the C3 UI.
If you need to roll back and C3 cannot find the installer in the download cache, it will be downloaded again.
I keep the most recent installer and a couple of the previous ones in the cache for quick roll back, the rest I store offline. I do this because I keep ALL installers used on my PC whether they are available via download or not.