Read https://trisquel.info/en/wiki/add-and-remove-programs about
installing/removing/updating packages. More generally, here is a list of
manuals: https://trisquel.info/en/wiki/all-manuals
As I wrote earlier, it is simpler to reinstall Trisquel (and, then, probably
the newer Linux-libre, like you have just done) setting up the proper
partition for /home (I imagine it is what you mean by /user, which does not
exist) right from the start, than do it afterwards (what involves copying
data from the current /home without altering the permissions, and editing
/etc/fstab).
So, first, you must make some space on one of your large disks. You can use
GParted to resize/move/delete partitions. You can keep using a live system.
Or you can install GParted on your new system and run it from there. But do
not touch the partitions the system is currently using (i.e., /dev/sdb1,
/dev/sdb2 and /dev/sdb5, if you run GParted from the newly installed)!
