Towards bottom of page you'll see Lightroom. Click on Buy button then choose Full or Upgrade
Hi. Be careful with the LR6 upgrade. I have done exactly that upgrading from LR5 on an iMac running Yosemite. I bought my upgrade from Adobe and downloaded it. I was sent the serial number and the software refuses to validate the number. Spoken to a real person at Adobe who would only confirm that the number is legitimate. All my LR software has always been bought from Adobe so is legitimate. Never had a problem upgrading or installing on two computers with previous versions.
Seriously considering returning the product and moving to OnOne perfect Suite as Adobe help is hopeless.
Abode help solution was correct. Hosts file filled with junk from previous installs. Reverting back to original hosts file allowed verification of serial number and installation of LR6.