We could do with the exact make and model of the router however you need to log in using the link you posted and enter the current user ID & Password. Then look for something like Router Management or Tools>User Management. There will be a place where you enter a new password twice to confirm and then apply/save the setting which usually reboots the router.
its an Orange, Siemens,Gigaset SE 572 WLAN dsl wireless router.
i clicl on my router admin in desktop & enter my password & i get a load of options, just can't see where an option is to change my current admin password! Not my usename & pass that is everything after the @ sign & password to bring up y account when i contact customer service, i mean the admin pass, that prevents people hacking my router!