It suddenly changed. The old version was DSC_XXXX. I want it this way; how can I change it back?
You changed from sRGB to Adobe RGB1998?
It's apparently the naming convention. It surprised me too when I did the same thing.
From my D5500 manual...
Photographs are saved using file names consisting of “DSC_” or, in the case of images that use the Adobe RGB color space (0 243), “_DSC”, followed by a four-digit number and a three- letter extension (e.g., “DSC_0001.JPG”). The File naming option is used to select three letters to replace the “DSC” portion of the file name.
The only way to revert is to go back to sRGB, or rename later in your computer.