Knowing what brand of router is being used would be helpful as most routers have instruction manuals at the maker's website.
However most routers have a reset button and depressing it and holding it will reset the router to defaults than allowing you to access the router's interface to configure the router, including log in (password and user id) for the router.
****edit*****
1. Open a webbrowser (e.g. Internet Explorer, Firefox or Opera). Enter the IP address of your router in the address bar of the browser and press enter. The IP address of your router is by default 192.168.1.1
Hopefully the address is not changed.
2. You see a box asking your username and password. By default the username for your ZTE ZXV10 W300 router is admin and the password is admin. If changed from the default hopefully you know current user name and password.
Enter your username and password and click the log in button to continue.
3. Assuming no DDNS Management, CWMP is NOT active go to Maintenance>admin to change the password
Should you need further support or information, check the link below for an PDF owner manual for the router.