Just figured it out here myself. I looked through the directories structure in appliance, and it looks most likely it's under the following mount point:
/storage/updatemgr
Now the next question is how can I remove individual rollups? lets say I have applied a 400MB Rollup, and it's no longer needed. How do I delete that individual 400MB rollup from the repository? or the only way is to reset the entire update manager?