Connect the printer to your computer via (do not use Wi-Fi for firmware updates to avoid connection drops).

If it is a zip file, extract it to a folder on your desktop. Run the executable file (e.g., ML-186xW_series.exe ). Follow the on-screen prompts.

These often involve dragging a .hd file onto a usbprns2.exe tool in a command-line interface.

Ensure the printer is "Ready" (green light) and has paper loaded. Download the firmware file (often a .zip or .exe ).

If you are looking for a "fix" (modified) firmware to reset page counts or use unchipped cartridges:

the printer or computer until the process is completely finished. The printer will usually reboot itself. Troubleshooting & "Fix" Firmware

Use HP Support Assistant to automatically find the correct software.