This is blatantly plagiarised from several sources but if I have it here I can find it again!
Using a Telnet program such as PuTTY or similar, telnet into the router. The default address would be 10.0.0.138 and the default user name is “admin” with a blank password.
Once connected, copy & paste in the following commands, one line at a time. Note that each line starts with a colon “:” The router’s “admin” user does not have sufficient privileges to add a root user, but it can add a script which when run can do the trick. I’ve stuck with “guru” for both user name & password as before, but you can choose your own …
I would suggest pasting into Notepad first to make sure the lines don’t wrap as they are displayed here.
:script add name addroot command "user add name guru password guru role root descr ROOT"
:script run name addroot pars ""
To test the new user, disconnect the Telnet connection and Telnet back in, this time using the user name “guru” and password “guru”. You should be able to get in.
Now using your web browser, go to your router at 10.0.0.138. Browse to “Toolbox”, then “User Management”, then “Switch to another user”. Enter “guru” and “guru” in the user name & password boxes. You should now be logged in as a root user. If you like you can change the default user by selecting “Set the default user” from the “User Management” page.
Note that a reset to defaults will wipe out this change. Also running the setup wizard restores some settings back to default – I’m not sure about the users.
Oh and the other thing you need is the CLI manual which you can get from here