I use slackpkg. It is quite smart and updates glibc-solibs and pkgtools first and then the rest, so there should be no problem. So here's how I do it (roughly):
0. backup everything important, Pat recommends /etc directory (+ your documents, of course
1. install slackpkg and configure it to use some of the -current mirrors (in /etc/slackpkg/mirrors and /etc/slackpkg/slackpkg.conf)
2. go to singleuser mode:
and make sure you have access to the network (ie use "ping google.com" or sth similar), if not, run /etc/rc.d/rc.inet1 start
3. run slackpkg update
4. run slackpkg upgrade-all
5. run slackpkg install-new
6. run slackpkg clean-system and choose the packages you want to remove (it lists all the installed packages that are not in official repositories)
7. make sure your symbolic links in /boot (System.map, config and vmlinuz) point to new files (and change it if you want to use generic smp kernel instead of huge one)
8. also make sure that /etc/rc.modules points to new rc.modules-'kernel-num' file
9. make initial ramdisk if you use reiserfs, raid or whatever that needs it (very good instructions in /boot/Readme.initrd)
10. run lilo and configure it to use the initrd and the new kernel
11. you can now try to reboot to see if everything works fine...
...then you would probably need to reinstall drivers for your graphic card and configure new Xserver, maybe uninstall kde internationalization packages, koffice or sth like that...
this is how I do it, I do not guarantee that this will work for you, it does for me.
I also recommend reading ftp://ftp.slackware.com/pub/slackwar...nt/UPGRADE.TXT
to see what are slackware official instructions.