Procedure below is copied from the PDF in the ZIP file that was downloaded from the TRENDNet download page for 4-Port Broadband Router TW100-S4W1CA (Version v2.0R), a discontinued model.
The ZIP Filename: FW_TW100-S4W1CA_v2(2.3.31).zip

And, note that the procedure in PDF was missing one very important caution, found on another TRENDnet router:

DO NOT upgrade firmware on any TRENDnet product using wireless connection.
Firmware upgrade over wireless connection may damage the product. Please perform firmware upgrade with “wired” network connection only

Firmware Upgrade Procedure 

1. Save the zipped firmware file to your PC.

2. Extract the .zip file to your PC.

3. Using a web browser, access the router’s configuration page (default IP address is 192.168.10.1)

4. Click on the System section in the left hand panel and click on Firmware Upgrade 

trendnet-firmware-upg

5. Click Browse and navigate to the unzipped folder with the firmware file.

6. Select the firmware file (.img)

7. Click the Apply button

8. Wait until the firmware upload completed and the device reboot

Important! Do not turn off the device or press the reset button while this procedure is in progress.


Ed.Note: copied this to make it more easy to find on search engines, to help people save time, to be helpful, that’s all.