There is no update to take you to LR 6.3 from LR 5.
LR 6 is a new version of Lightroom so you will have to purchase and upgrade to install and register.
If you have a Creative Cloud subscription the you can download Lightroom CC through your Creative Cloud account. LR CC 2015.3 is the same as LR 6.3 with a couple additional features.
Ok thanks. I thought it was just part of the updates with having purchased CC which I pay for each month. Cheers
If you are a Creative cloud customer, you can download and upgrade your Lightroom 5 to Lightroom CC 2015 without any additional charges.
Hope this helps.