First of all: backup the user data!
The easiest solution is to reinstall. When the installer asks for the type
of install, choose "Other" (or whatever it is called nowadays). Then, within
the partition editor integrated with the installer, delete the existing
partitions, recreate a larger partition (say 48 GB to be at ease) with a
filesystem (ext4 is fine) to be mounted at /, a swap partition (as large as
your RAM if you want to hibernate, otherwise smaller), and another partition,
taking the rest of the disk, with a filesystem mounted at /home.
For that last filesystem, Trisquel chooses XFS by default. It is fine. In
particular, XFS is efficient with large files, such as user files. But a
drawback is that XFS cannot be shrunk. That is because I imagine your /home
is on XFS that I suggest you to reinstall. Because you cannot shrink it, you
would have to delete it, enlarge the root partition, recreate it, and edit
/etc/fstab. All from a live system. You can try that though, if you want to
learn.
Notice that you need not lose all the work you put into installing packages
on top of the default system. See
https://trisquel.info/en/wiki/cloning-system-or-how-make-copy-installed-packages-one-computer-another
