https://docs.slackware.com/slackware:beginners_guide

Before you can use slackpkg you will need to define an online mirror from which it will download updates to your computer.

A list of available mirrors for your Slackware version can be found in this file from /etc/slackpkg/mirrors

…. via Terminal

su

nano /etc/slackpkg/mirrors

… uncomment only one URL

…. save and quit

Ctrl o
Ctrl x

Installing updates using slackpkg
The slackpkg update command will connect to a Slackware mirror and update the local package information database on your computer. This command does not actually install any package!
The usual routine for upgrading your Slackware to the latest patches is as follows:

slackpkg update
slackpkg install-new
slackpkg upgrade-all
slackpkg clean-system