D-Link DIR-400 is a rather old router model.
I will describe the steps in steps:
1) Download the new firmware from the official site
2) Open the router settings by typing the address http://192.168.0.1 in the browser (maybe 192.168.1.1) and enter the standard login – admin without a password.
3) In the interface that opens, select the “MAINTENANCE” tab, to the left of the “Firmware Update” menu. Then, where “UPDATE SETTING” click “Browse” and select the previously downloaded firmware file, then click “Apply” to start the update process.
We are waiting for the completion of the update, usually 2-5 minutes. At the end, the router will restart itself.