Here is how:
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Duplicate your main theme.
Go to Themes → Theme Manager. On the Theme Manager page, click on the 'Duplicate theme' link for your Main published theme. That's in the top section under the heading 'Published Themes'.
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Edit your duplicated theme. Preview your changes.
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When you are done, publish your duplicate theme as the main theme.
Afterwards, you can keep the old copy of your theme or delete it.
Note: You can rename a theme on the Template Editor page:
