Thank you for the question. When Mavericks is already installed on your computer, you can reinstall using the Recovery partition on your iMac:
OS X Mavericks: Reinstall OS X
Use the built-in recovery disk to reinstall OS X while keeping your files and user settings intact.
Important: You must be connected to the Internet to reinstall OS X.
- Choose Apple menu > Restart. Once your Mac restarts (and the gray screen appears), hold down the Command (⌘) and R keys.
- If you’re not connected to the Internet, choose a network from the Wi-Fi menu (in the top-right corner of the screen).
- Select Reinstall OS X, then click Continue.
- Follow the onscreen instructions. In the pane where you select a disk, select your current OS X disk (in most cases, it’s the only one available).
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Thank you for the additional information. Due to the nature of your issue or question you may find more information by using another one of Apple's support resources - https://getsupport.apple.com/GetproductgroupList.action
All the best,