Thank you guys. Managed to solve it by:
If you don't want to use slackpkg you can download the packages from the k/ directory on one of the slackware-current mirrors and upgrade with the `upgradepkg` command
I downloaded from a directory all the kernel files.
Step 1: rsync -av rsync://taper.alienbase.nl/mirrors/slackware/slackware64-current/slackware64/a/kernel-* .
Step 2: cd to current directory
Step 3: pkgtool
Step 4: select "install packages from current directory"
Step 5: lilo
In my case, i boot from a USB Boot stick, so i need to recreate the boot stick
Step 6: pkttool, select SETUP Choose Slackware installation scripts to run again
Step 7: select "Create a USB Linux boot stick"
Step 8: Exit from pkgtool, then Reboot