Upgrade Roadmap for CE 1.9.0.1 from 1.8
- take a backup of current database and current 1.8 code.
- download latest magento from the http://www.magentocommerce.com/download
- remove all folders and files from your 1.8 code (but you should have backup somewhere) and place all folders and files from the 1.9.
- now from your 1.8 merge your following folders into the 1.9
- Community app/code/community
- Local app/code/local
- Media
- your theme or package (app/design/frontend/default/ or app/design/frontend/)
- custom folders from Skin (both for adminhtml and frontend).
- copy your custom xml files from app/etc/modules/ to current app/etc/modules/
- any custom admin theme folder from adminhtml/default/yourtheme.
- copy your custom folders from adminhtml/default/default/ (1.8) to adminhtml/default/default/ (1.9).
- custom js files if any from app/js/.
- now go to app/etc/local.xml.Edit database details their.put your database username and password and database name.
- now check the site.it done.