Did it still gave you the same error after doing "apt-get update"? It's
weird. As far as I know all what it means is that the source of the package,
AKA where is it coming from, can not be verify, which means that some one
could be trying to fool your system to think that you are receiving the
package from trisquel but you are not, and instead you end up installing
malware.
This isn't usually the case though. I don't know much about certificates to
tell you the true, but things like your system time and date being wrong can
cause problems with the verification. Most other problems get fix with
"apt-get update" in my experience. Some times waiting an hour also help.
Did you have other packages that needed upgrade? Do those packages didn't
give you problems? Usually all packages coming from the same source would
give you problems if there are difficulties verifying the source. Maybe you
have add other repositories? You can check where the system got the package
by going to the package properties on synaptic.
If you didn't added other repositories I wouldn't worry too much if you
already upgraded it. I haven't hear of any trisquel user been attack through
package upgrading, and most of the time it's just an error with the
verification process. It's better to be safe though.
Try to install anything else coming form trisquel's repository and see if it
gives you any error. It can be anything like a game called Majicor that I
love so much. Then if one step before you install it it gives you also the
same error post the entire output to see if some one in the forum can help.
