Removing Debian GNU/Linux from my Trisquel Machine

I'm assuming you want to delete the partition Debian is on, and give that space back to another partition? There is documentation in the Trisquel manual for doing this with Parted (https://trisquel.info/en/wiki/how-format-external-storage-device-using-parted) but you can also use GParted, which provides a GUI for Parted. GParted can be installed in Trisquel from the repos (using 'sudo apt-get gparted' or Synaptic or Add/Remove Applications), but I find it's less complicated to use GParted by booting from a Trisquel liveCD/DVD/USB (eg the one you installed from). GParted is under 'System Settings'.

Be *very careful* with GParted! Make sure you know exactly which partitions you want to remove, and which ones you want to keep, and resize. It's a good idea to back up any mission critical data first, on a drive which isn't going to be in your computer when you used GParted.

For next time, there are a few ways to repair GRUB, other than installing another version of GNU/Linux. If you can boot into the install you want to keep, it usually works to do that, login, open a terminal (CTRL + ALT + T) and run this command:
sudo update-grub

That command will instruct GRUB to re-check your hard drive for OS partitions and add them to the boot menu (it also removes any OS no longer present).

If you can't boot even far enough to enter commands in a terminal, you can fix GRUB using the same liveCD/ DVD/ USB you used to install your GNU/Linux system (or another one will usually work too). I found these instructions very thorough, and easy to follow:
http://ubuntuforums.org/showthread.php?t=1581099

BTW There's an ongoing thread on GRUB issues here, which covers other problems such as random GRUB passwords stopping you getting into pre-existing OS after a Trisquel install:
https://trisquel.info/en/forum/grub-issues