Go in your internet browser and type 192.168.1.1 in the same box you type a website link in (ie google.com)
Sign in with the username and password (typically listed on a sticker on your router)
Take a look through all the menus until you find a menu allowing you to modify the password. A google search may help with this like "how to change password on [insert company name here] router".
Note: Any device on the network will be kicked off and need to be reconnected with the new password when you change it.