slackpkg is a very convenient tool for that job. It can download update packages from the net and install them. In order to use it, you first have to uncomment a Slackware mirror close to your location for your Slackare version in the file /etc/slackpkg/mirrors

( ftp://ftp.gwdg.de/pub/linux/slackware/slackware-current/ works good for Germany )

Slackpkg Usage:

rm -r /etc.bak;        # Back up /etc to be on the save side
cp -a /etc /etc.bak
slackpkg check-updates # to see if updates are necessary at all
slackpkg update        # to update the package list
slackpkg upgrade-all   # to finally install the new versions of existing packages

Go on with fixing config files (below)!

rm -r /etc.bak;
cp -a /etc /etc.bak
slackpkg check-updates
slackpkg update
slackpkg install-new
slackpkg upgrade-all
slackpkg clean-system

Whole package series can be blacklisted as well, eg

./slackware64/kdei

blacklists everything from the kdei series (tested). See also

Go on with fixing config files (below)!

Make sure to have a copy of the original version of every file you modified. Give the original files a uniform extension, eg. .sborg

Make sure you don't have any old .new files somewhere in /etc

After slackpkg has installed the new packages you will be asked how to treat the new config files. Select the first option, i.e. old config files are preserved and new config files get the extension .new.

For everey new config file, it is most convenient to do an automated mv or merge with the following script:
Link

/var/www/htdocs/index.html may be created by the update if it does not exist. This may conflict with your custom start page, eg index.php

Your php.ini may need manual treatment. If your php.ini is based on php.ini-production, you have to check if your model php.ini-production was modified by the update and merge the differences into your php.ini. This is because currently (Slack 13.37-14.1) no .new files are created for PHP.

Recommended approach for php.ini:

Afer OS installation, before any OS update:

mv php.ini-production php.ini.sborg
cp php.ini.sborg php.ini
echo ';Renamed php.ini-production to php.ini.sborg for updates' >> php.ini
echo ';This file php.ini is based on php.ini.sborg' >> php.ini
# now furhter edit php.ini to satisfy your needs

After OS update, but before running the sbMergeEtcConfig script, a (new) php.ini-production may exist again. Then do:

mv php.ini-production php.ini.new

Now run sbMergeEtcConfig

Have at least 2 partitions: A /-partition where the linux distribution is installed and a /home where all distribution independend things are located. During upgrade, / will be completely overwritten by the new distribution, while /home remains untouched.

Not tested yet: If your current system isn't split like this, you can copy the content of the disk to an image file on a separate drive (eg. USB-drive), then re-partion the drive and restore the content afterward. Use commands like:

dd if=/dev/hda1 of=harddisk-image # Make the image
dd if=harddisk-image of=/dev/hda1 # Restore the disk content

See http://ldp.bootet.net/LDP/sag/html/sag.html#DISK-NO-FS . Maybe also the cat command is an option. See slackware-11.0/rootdisks/README.TXT

We assume that /home has its own partition which will be conserved during upgrade. Everything which has to be saved and is locatet in the / partition has to be moved and symlinked to the /home partition:

mkdir /home/redirects
 
cp -a /usr/local /usr/local.org
mv /usr/local /home/redirects/usr.local
ln -s /home/redirects/usr.local /usr/local
 
cp -a /opt /opt.org
mv /opt /home/redirects
ln -s /home/redirects/opt /opt
 
cp -a /var/www /var/www.org
mv /var/www /home/redirects/var.www
ln -s /home/redirects/var.www /var/www
 
# as of Slack 12.2 you might update the symlinks for the apache document root as well:
cp /srv/httpd /srv/httpd.org
cp /srv/www   /srv/www.org
ln -s /home/redirects/var.www /srv/httpd
ln -s /home/redirects/var.www /srv/www

Thest if your moved data and applications are still available and working after that step!