There are two starting points. If your boot partition isn't very large, you'll get errors when installing the linux-lts package. Example:
zstd: error 25 : Write error : No space left on device (cannot write compressed block)
bsdtar: Write error
bsdtar: Write error
==> ERROR: Image generation FAILED: sort reported an error
error: command failed to execute correctly
So I just deleted all of the .img files in /boot
sudo rm /boot/*.img
Now install the LTS kernel. If you reboot before installing the new one, your system will not boot.
sudo pacman -Syu linux-lts linux-lts-headers
If using VirtualBox, you need to swap its kernel modules to match the new LTS kernel before you can remove the Linux package.
sudo pacman -S virtualbox-host-dkms
Allow the replacement.
virtualbox-host-dkms and virtualbox-host-modules-arch are in conflict. Remove virtualbox-host-modules-arch? [y/N] y
Now remove the linux and linux headers packages
sudo pacman -R linux linux-headers
Finally, update your bootloader.
sudo grub-mkconfig -o /boot/grub/grub.cfg