I've got a bit of a problem on my computer since the entry for lubuntu decided to vanish for some reason. I'm running lubuntu 17.10.1 on a Asus 1011PX. I'm thinking I could just change one of the entries so it boots lubuntu instead of memtest or ipxe. Lubuntu is installed on sda1, no encryption if that helps. I did update recently before this happened so that might have something to do with it.
If you really want to edit one of the existing entries, you can follow this askubuntu question for help. It also explains how to add a new entry.
However, I would recommend figuring out why your grub config was recreated without the lubuntu entry. Use
sudo grub-mkconfig -o /boot/grub/grub.cfg.new to find out what entries are currently detected. The file
/boot/grub/grub.cfg.new will then contain the configuration as detected. If you are happy with the configuration, move it to
/boot/grub/grub.cfg for the changes to take effect.
Ubuntu provides the command
sudo update-grub if you simply want to regenerate your grub configuration.
If you are currently unable to boot into lubuntu or another linux, follow this askubuntu answer to repair grub entirely, including rewriting the MBR.