If you hover over the setting that you want to change, you will see a little pencil. Search in the bar for default and scroll down until you see file However, if you just want your new file to open as a particular language every time you can change this in the settings. Then you can choose the language for the file.
That’s CTRL and K together and then M afterwards separately.
To change the language for the file use CTRL +K, M.
When you open a new file in VS Code (Using CTRL + N) it opens by default as a plain text file.