Note, for syslinux you'll need to edit the syslinux config file in /boot/syslinux/syslinux.cfg accordingly, just point everything to the -lts kernel.
It seems you are using systemd-boot as bootloader. syslinux would be another alternative but you could have inferred (from the other answers to that question) that this also applies to your situation if you are not using GRUB.
To fix your boot issue you should make the appropriate modifications in the entry /boot/loader/entries/arch.conf (i.e. append -lts to linux /vmlinuz-linux and insert -lts into initrd /initramfs-linux.img).
To access the file you can simply mount the EFI system partition from any live environment (i.e. the Arch Linux installation CD/USB).