root@host:~# slackpkg install-new

You need the GPG key of Slackware Linux Project <security@slackware.com>.
To download and install that key, run:

        # slackpkg update gpg

You can disable GPG checking, too. But it isn't a good idea.
To disable GPG, edit slackpkg.conf and add one line with NOGPG=1.
You can see an example in slackpkg.conf.new.