Can I install Lubuntu without having the admin password?
Yes. You don't need to remember old Ubuntu password to install new. You just boot a bootable CD/USB and wipe the partition belonging to old Ubuntu install and install Lubuntu on it.
If you still want to reset the password check this question
- How do I reset a lost administrative password?
To create bootable USB from ISO check this question
- How can I create a Windows bootable USB stick using Ubuntu?
Check these for installation help
- https://help.ubuntu.com/community/Lubuntu/InstallingLubuntu
- How do I install Ubuntu?