If Adobe releases a new version of LR you should get a notification from the software but you can also check Help->Updates within Lightroom for updates and upgrade options.
Updates to a specific version are free, but you'll have to pay to upgrade to a newer version (e.g. v5.3 to v6.0). ....
I downloaded the free version a few weeks ago. It is rendering very slow for me. Even with simple tasks like highlights and exposure. I have a pretty fast PC with an SSD drive and my LR/PS run pretty fast on it.