If you have more information about where you saw this file, I can provide a more tailored answer. If you can provide more context, I can give better advice: Where did you this file? Is it a file you are trying to fix or extract ?

You will need a file archiver tool. The most common and recommended tool is 7-Zip (free, open-source).

download or open the file unless you are absolutely certain of the source.

If you are asking how to open a 7z file in general, it is a high-compression file format. Here is how to handle it:

the file with an updated antivirus program (like Windows Defender, Malwarebytes, etc.) before attempting to open it. General Guide to .7z Files

WinRAR or PeaZip can also handle this format. Instructions: Download and install 7-Zip. Right-click the derjrefkio.7z file. Select 7-Zip from the menu. Choose "Extract Here" or "Extract to [Folder Name]" .