Boot from the Linux Mint installation DVD / USB. Open GParted from there. You can then delete the Lubuntu partitions, and resize the Linux Mint partitions to reclaim the free disk space.
If you Linux Mint partition is at /dev/sda1, you can do the following to reinstall GRUB. So you are sure that the bootloader is loading GRUB from the Linux Mint partition and not trying to load it from the now non-existent Lubuntu partition. You can skip this step if you are sure Lubuntu didn't overwrite the bootloader (for example if you told its installer to not write it to /dev/sda but somewhere else). Change /dev/sda1 in below first command with the /dev/sda? name of your Linux Mint partition if that is different.
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sudo mount /dev/sda1 /mnt
sudo grub-install --root-directory=/mnt /dev/sda