To set up or change default gateway, you can use ip command, route command or you can manually edit /etc/network/interfaces file.
Option 1: ip command
ip route add default via 10.0.0.1
Option 2: route command
route add default gw 10.0.0.1
To delete the default route,
route del default gw 10.0.0.1
Option 3: Save routing information to a configuration file /etc/network/interfaces
Open /etc/network/interfaces file
Find network interface and add the following option
... gateway 10.0.0.1 ...
Save changes and restart networking
In case you don’t have route command, you can get it via net-tools package.
sudo apt install net-tools