Any existing Cubase 11 LE users are entitled to upgrade to Cubase 12 LE for free. You should have received an email from Cubase shortly after the release of Cubase 11 LE that outlines the steps to update.
More information from Steinberg about this can be found here: https://helpcenter.steinberg.de/hc/en-us/articles/4448449036946-Cubase-LE-AI-12-free-updates-for-users-of-previous-versions-