How to upgrade from Trisquel mini to full

Yes, by installing the "mate-desktop-environment" package, e.g., using the "Synaptic Package Manager" or 'sudo apt install mate-desktop-environment' in a terminal.

The "trisquel" and trisquel-recommended" (meta)packages actually define Trisquel's whole default system. You may want to take a look at their dependencies (again using the "Synaptic Package Manager" or 'sudo apt show trisquel trisquel-recommended') to discover additional applications you may want to install. You can even install the metapackages. You would however end up with two programs for most tasks: that of Trisquel and that of Trisquel Mini. If your goal is to end up with the sole Trisquel's default system, installing it from scratch is probably faster than figuring out and removing all packages that are in Trisquel Mini but not in Trisquel (although that is not impossible).

Thank you, I just wanted to check how MATE looks and feels like. Your answer worked like a charm, thanks again.