You didn't say where you are. In the USA and the Adobe system, it is common now to use the Creative Cloud plan at $10USD a month. You'll get the current version of LR CC 2015.4
If you work through the Adobe online purchase system for LR 6.4, at the very end is an option for an upgrade discount.
The first step is making sure your computer is compatible. LR 6 is 64-bit only.