Once I needed to change the default gateway on different servers without restarting the network service or rebooting the server, I will give examples of how I did this.
First look at the current routes:
ip route
On Ubuntu Server I added a new gateway and then removed the old one:
ip route add default via 192.168.5.8
ip route del default via 192.168.5.1
On a server with Proxmox based on Debian, this did not work out, an error was displayed:
RTNETLINK answers: File exists
So I ran the command (this is a more correct option):
ip route replace default via 192.168.5.8
Then I changed the gateway in the network configuration file so that the changes would not be reset after restarting the server:
cd /etc/network/
nano interfaces
If the server uses Netplan, you can change the gateway in its configuration and then run:
netplan try
netplan apply
How to change other network parameters, such as IP address, etc. look at my articles:
Configuring the Network in Linux
How to configure networking with Netplan