It’s relatively simple:
sudo pacman -Syu linux-lts linux-lts-headers
Then do:
sudo grub-mkconfig -o /boot/grub/grub.cfg
Next time you boot up, you will be able to select it in the boot menu.
The above two steps are all you NEED to do, so you can stop here. The below information is to customise how things work.
Note: Installing a new kernel normally causes Grub to select the new kernel as the default kernel, so you may need to edit the file below to prevent this from happening.
https://wiki.archlinux.org/title/GRUB/Tips_and_tricks#Multiple_entries
sudo nano /etc/default/grub <- This opens the file as root
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT=true
-> The above settings saves your selection in the Grub menu as default
Optionally, you can remove submenus by editing the line below like this:
GRUB_DISABLE_SUBMENU=y
If you do any of the extra steps, then re-run the command below after:
sudo grub-mkconfig -o /boot/grub/grub.cfg