One of my clients is setting up a multi-store instance of Magento - and wanted to change each stores home page. I have not used Magento in a multi-store instance before so I had to look up the details.
Ok - here is what you want to know - I updated one of the stores - the required steps as follows
1) create a new page in the cms (named something like store-home)
2) If you have not already, create the new store
3) in admin open menu System->configuration
4) select box at top left - choose the entry 'english' under 'Maria's Botique'
5) left menu - choose 'web'
6) on right choose 'default pages'
7) modify the 'cms home page' by choosing the custom page we created in step 1
8) done
My reference material can be found here
http://www.magentocommerce.com/knowledge-base/entry/overview-how-multiple-websites-stores-work
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