under netwock choose your network interface and klick on it in the list, a configuration menu will pop up on the right side of the screen.
Uncheck the dhcp checkbox and scroll to the bottom. There's a menu point "IP Adresses", add the ip you want to add.
Next you have to manually configure your nameservers and default route(router ip), thats also under "network but global configuration"
There's a "setting" button on the right side of the screen.