If you can connect to the router through LAN cable or wireless and access its webpage (mostly http://192.168.0.1 or you may have to check the product box for the default) and you already know the password you can reset from the reset page. This is safe normal process and it will only reset the password.
Now if you don’t have the old password and no access to router page, you will have to go for the hard reset using the reset button, you can press and hold for few seconds (6-10 secs). The button will be some where behind the router or underneath and it will be small one that you may have to use sharp objects to press. This will reset all the custom router settings to factory setting.