How do I turn off Ubuntu interface?

if you want to disable for example the eth0 (ethernet port), you can sudo ifconfig eth0 down which will disable (down) the port. changing down to up will re-enable it. use ifconfig to view your ports.

Which Linux command is used to bring the interface up?

Linux ip Command with Examples. The ip command is a powerful tool for configuring network interfaces that any Linux system administrator should know. It is used to bring interfaces up or down, assign and remove addresses and routes, manage ARP cache, and much more.