For example, I will update the D-Link DSL-2600U modem with firmware 1.0.1 to version 1.0.5.
I will describe the steps in steps:
1) Download the new firmware from the official site
http://www.dlink.ru/ru/products/2/1518_d.html
2) Open the settings of the router by typing the address http://192.168.0.1 in the browser (maybe 192.168.1.1) and enter the standard login – admin, the password – admin.
3) In the interface that opens, select “System” – “Software Update”, then click “Browse” and select the firmware file previously downloaded, then click “Update” to start the update process.
Wait until the update is complete, usually 2-5 minutes. At the end, the modem will reboot.
Done.