I am new to magento. I want to add a sub category to top nav bar. For that i added necessary subcategory under manage categories. But i cannot see this category in top nav bar. Please help me.
You will want to double check that the new sub category is enabled and has "include in navigation" set to yes. After that, then all you should need to do is reindex. It's unlikely that you'll need to flush cache but depending on your setup you can do that as well.
Sub category is active and "include in navigation" set to yes . Cache is disabled. Which indexes should i re-index? all of them? "Product FLAT data" index always shows "reindex required" .
Are you using Magento 1 or Magento 2?
Not sure precisely on which index, there are a few related to categories/catalog. I'd just make sure that all of them had been updated recently.