How do I change the name of the "Uncategorised" category

The only way to edit that category's name is through the language files. There are two methods you can use to achieve this:

Manual

Let's say that you are using the standard English (en-GB) language in your website's backend:

• Head to /administrator/language/en-GB and edit the en-GB.com_rsblog.ini file.
• Look for the following line: COM_RSBLOG_UNCATEGORISED="Uncategorised".
• Edit the label (contained within the ,", characters) to whatever suits your needs.

Using Joomla!'s Language Overrides feature

Joomla! 3.x includes, by default, a very useful language override feature that can save you from a lot of trouble when editing the language files:

• Head to Extensions >> Language Manager >> Overrides
• Click on the New button to the upper-right side of the screen
• In the Search text you want to change area (right side of the screen), select Constant
• Type in COM_RSBLOG_UNCATEGORISED in the Search field and click on Search
• Click on the search result below and head to the Create a New Override area to the left side of the screen.
• Type in your desired label in the Text field and click Save.