Recently, my PC got infected with a ransomware virus, which has changed all files to extension .derp. Now, how do I change all the file extensions to their previous versions?

To revert the file extensions changed by ransomware (.derp in this case) and attempt to recover your data, you can try the following steps:

1. Disconnect from the Internet: First and foremost, disconnect the affected device from the internet to prevent further encryption or spread of the ransomware.

2. Use Antivirus Software: Run a reputable antivirus or anti-malware program to scan and remove the ransomware from your system. Make sure the software is up-to-date for better detection rates.

3. File Recovery Tools: Try using data recovery tools like Recuva, EaseUS Data Recovery Wizard, or PhotoRec to recover deleted or encrypted files. These tools might help in recovering some of your original files.

4. Check for Backup: If you have a backup of your important files, restore them from the backup after ensuring that the ransomware is removed from your system.

5. Decryption Tools: Check online resources like the No More Ransom Project, which offers decryption tools for some types of ransomware. Be cautious and ensure you download such tools from legitimate sources.

6. Consult Experts: If you have valuable data or if the above methods do not work, consider consulting with a professional data recovery service or a cybersecurity specialist for further assistance.

Remember to regularly back up your data to avoid such situations in the future.