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