If you would like to install or upgrade a package that you have manually downloaded or have built yourself, you can use spkg. Salix packages (just as Slackware packages) have filenames that end with the .tgz, or .txz extensions (there are also .tlz and .tbz but they are very rarely used). To either install a new package or upgrade a package you can use the following command:
sudo spkg [exact path to package]
Note that the name of the package must be exact to the last character in the extension, e.g. mozilla-firefox-24.3.0esr-x86_64-2gv.txz (command line auto-complete using the TAB key helps in this respect). Removing a package is somewhat similar. You can remove a package by issuing
sudo spkg -d [software name]
e.g. sudo spkg -d skype