If the file is missing from your system, the safest way to recover it is to repair your Office installation
Assuming you have the file in the correct location, here is how to use it properly. Download Ospprearm.exe
Some fake "download" sites ask for your Microsoft account or admin password under the guise of "verification". If the file is missing from your system,
The rearm operation can only be performed up to 5 times. After that, you must enter a valid product key. After that, you must enter a valid product key
On a standard installation, you can find the file in the Office installation folder. The path depends on your Office version and system architecture:
If the file is missing, run an Office repair. If Office isn’t installed, install a legitimate volume license version from Microsoft. And if you only need to reset the activation timer, use ospp.vbs /rearm instead.