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I assume you want to run a new XOOPS installation on the subdomain. In that case upload a new set of XOOPS files to the folder to which your subdomain points, and run the installation script from your subdomain.
For example: example.domain.com usually points to a folder in your main folder, so probably a folder named 'example' can be found in your main folder. In that case you would upload your XOOPS files to the 'example' folder.
Remember to use a different prefix for your XOOPS tables if you use the same database as your main XOOPS site.
Martijn