LR 6 is a new payfor upgrade, $80 in the US if you have the serial-number-licensed LR 5. If you're already using the CC version of LR+PS then it just comes along as a new app you can install.
It’ll run alongside your LR5 so you can try it out and see if it’s something you want.
The only trial is a LR-6-CC version that you can convert to a serial-number-licensed version if you buy a serial-number. I think the download in LR 5 when it notifies you of an upgrade is what you can use to try it, although I don’t have personal experience with that seeing as I have the CC version of LR+PS so the CC App provided the Install button for me.
The main issues people are having are for some system configurations performance is worse. Adobe rewrote it to use the GPU in the Develop module but some people’s video cards aren’t up to the task, and for some people even turning off the GPU, while speeding things up, doesn’t make it as fast as LR 5.
On my system it seems to work great.
The Adobe web site does not make it easy for users of stand-alone LR5 to upgrade to LR6 who have to navigate through all of the Creative Cloud advertising.
Look for 'Products' at the bottom of the adobe.com home page then toggle 'Full' to 'Upgrade' before you 'Buy' Lightroom 6.
For me this went from $A186 to $A99 (Australian dollars). I use LR for all my photo library image manipulation because it is IMHO the best product out there and I'm looking forward to the new features in LR6. If only they would improve the slide show module or integrate with FotoMagico...
Use this link and click the Buy button. Then click the dropdown to change from Full to Upgrade and then choose your present version (see image) and the upgrade price should be displayed. Then click “Add to Cart” You can download from here