我想从 16.04 升级到 20.04，并经过 18.04。但是，当我尝试更新到 18.04 时，我收到：
The upgrade has aborted. The upgrade needs a total of 5.754 M free space on disk '/'.
Please free at least an additional 3.059 M of disk space on '/'.
以下是输出df -h：
$ df -h
Filesystem Size Used Avail Use% Mounted on
udev 3,9G 0 3,9G 0% /dev
tmpfs 789M 9,4M 779M 2% /run
/dev/sda5 19G 15G 2,6G 86% /
tmpfs 3,9G 92M 3,8G 3% /dev/shm
tmpfs 5,0M 0 5,0M 0% /run/lock
tmpfs 3,9G 0 3,9G 0% /sys/fs/cgroup
/dev/loop1 55M 55M 0 100% /snap/core18/1668
/dev/loop0 161M 161M 0 100% /snap/gnome-3-28-1804/116
/dev/loop2 28M 28M 0 100% /snap/snapd/6953
/dev/loop5 25M 25M 0 100% /snap/snapd/6434
/dev/loop4 59M 59M 0 100% /snap/discord/108
/dev/loop3 55M 55M 0 100% /snap/core18/1705
/dev/loop8 45M 45M 0 100% /snap/gtk-common-themes/1440
/dev/loop7 58M 58M 0 100% /snap/discord/109
/dev/loop6 49M 49M 0 100% /snap/gtk-common-themes/1474
/dev/sda6 421G 351G 49G 88% /home
/dev/sda1 496M 66M 431M 14% /boot/efi
tmpfs 789M 64K 789M 1% /run/user/1000
考虑到（我想）我以后需要更多空间来升级到 20.04，最好的解决办法是什么？