Hi Max, I’ll translate my previous message hoping that it will help you. If you have more questions please just ask.
My previous post now in English:
First, that update is recent, so some mirrors are not updated yet, but others are. I’ve tried and it worked here.
Try to update first your pacman db and then install the package:
pacman -S intel-ucode
Oh, to install a downloaded package you don’t have to decompress it, just run:
pacman -U package.xz
The second step is a bit of a botched job, as it tells you to edit the grub.cfg file (which in normal circumstances you should not do, as it’s an automatically generated file by the command grub-mkconfig).
So, edit the grub.cfg file (/boot/grub/grub.cfg) and just add /intel-ucode.img to the initrd line, so it looks like this:
initrd /intel-ucode.img /initramfs-linux.img