To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.
lubuntu@lubuntu:~$ sudo mount /dev/sda4 /mnt/
lubuntu@lubuntu:~$ ls -l /mnt/
total 20
drwxr-xr-x 26 1000 1000 4096 Aug 11 13:05 jari
drwx------ 2 root root 16384 Jan 22 2021 lost+found
lubuntu@lubuntu:~$ findmnt /mnt/
TARGET SOURCE FSTYPE OPTIONS
/mnt /dev/sda4 ext4 rw,relatime
lubuntu@lubuntu:~$ sudo umount /dev/sda4
lubuntu@lubuntu:~$