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You will need to upload your theme via FTP.
In FTP, navigate to yoursiteroot/themes - here you should see 3 folders. The 'default' folder contains your default theme. You can overwrite the theme.html file in this folder with your own, or you can create a new theme folder in your /themes/ directory & upload your theme files to that location.
If you create a new folder in the themes directory with your new theme files, you will then need to navigate to your XOOPS site admin > Preferences > General and select your new theme in 2 places - under 'Default Theme' and under 'Selectable Themes'. Also set "Check templates for modifications?" to Yes.
Hope this helps!