I will describe the steps in steps:
1) Download the new firmware from the official site:
http://ftp.dlink.ru/pub/Router/DIR-620/Firmware/.
Carefully look at the revision on the label of the router, since each uses its own firmware. Flashing the wrong firmware can damage the router.
2) Open the settings of the router by typing in the browser address http://192.168.0.1 (maybe 192.168.1.1) and enter the standard login – admin, the password – admin.
3) In the opened interface, select “System” – “Software Update”. Next, click “Browse” and select the previously downloaded firmware file, then click “Update” 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.
Done.