You haven't specified the Router model, but I'll try to help!
Open your web browser and type your router Ip (if it is 192.168.0.1 or 192.168.100.1) It will ask you for username and password... (by default it is: admin - admin, admin - 1234 of course it is router model sensitive) Log in and then find find "Local Network, Local IP, Lan" and change the current subnet with new... Maybe my answer is not exact it because I don't know your router model, brand
With best wishes