How to: Change Ubuntu Desktop from DHCP to a Static IP Address
https://haxordoubt.blogspot.com/2013/04/how-to-change-ubuntu-desktop-from-dhcp.html
If you’ve installed your Ubuntu installation with DHCP, it’s really easy to reconfigure your system to use a static IP address with the GUI based Network Settings dialog.
To open the utility, go to System \ Administration \ Networking
You’ll see your network adapter in the list. Click the Properties button, and you’ll see this dialog:
Just change the drop-down box to Static IP address, and then put in your static settings