If your Mac is refusing to let you delete files from /Library/Update/ even with admin privileges, it’s likely due to System Integrity Protection (SIP) or locked file permissions. Since macOS Catalina (10.15), the system volume (Macintosh HD) is mounted as read-only, meaning no user—including root—can modify or delete files directly within /System, /bin, /sbin, or /usr (except /usr/local). /Library/Update is in this system volume, as well.
FYI, the root user has "higher" permissions than the Administrator user. However, the root user is disabled by default.
You can verify the current permissions on this folder by using the following command in the Terminal app:
Most likely, you will find these permissions are assigned to the root user ... a user account that is no longer easily accessible with newer versions of macOS.
Is it possible to delete files in the /System/Update folder? Yes, but it requires both enabling the root user and temporarily disabling SIP. Neither are recommended.
Warning: Modifying system files can cause macOS to become unstable or prevent updates from working properly. If you're trying to remove old update files, a safer approach is to let macOS handle them automatically or use Disk Utility to clear space on the user-accessible Macintosh HD - Data volume.