If you format a partition (i.e., "install" a filesystem on it), its files are
lost.
However, some filesystems can be shrunk and/or enlarged without losing
anything. E.g., the ext family of filesystems support both operations (but,
of course, it will not let you shrink it to a size where there is not enough
space for its files). XFS (Trisquel's default filesystem for /home) can only
be enlarged, not shrunk. So, to effectively shrink an XFS filesystem, you
need to backup its files (on another filesystem), delete its partitions
recreate a smaller XFS filesystem (or you can choose another one) in the free
space and move the file that were backed up on the newly created filesystem.
GParted (a graphical interface to GNU parted) makes it easy to alter your
partitions and their filesystems. It will warn you if the operation you plan
to execute will make you lose the files on some partition. GParted is on
Trisquel's live system. You should never alter partitions that are mounted,
i.e., with their files accessible. For such partitions, you need to run a
live system.
