Then maybe the downloaded packages are actually corrupted. You could check if they have plausible file sizes. IIRC pacman will ask you if you want to delete the non-matching files but I’m not entirely sure. They should end up in /var/cache/pacman/pkg.

Update only archlinux-keyring and try again.

# pacman -S archlinux-keyring
# pacman -Syu

In some cases you may need to re-populate the keyring.

# pacman-key --init
# pacman-key --populate

Adding to this: individual package updates are not supported. Always update your whole system, unless you know what you are doing.