In Magento 1.9 » Navigate to “app/design/frontend/default/themeXXX/locale” folder, where themeXXX is your actual theme name.
Click the language file, usually English is en_US
Open translate.csv in edit
Add "Categories","Shop Now" to end of file. You can replace the “Shop Now” with any text to your liking.
Click here for the video demo.