You can buy the upgrade and it will install version 6 (with your version 5 serial number making you eligible to pay the lower upgrade price).
Only those who have never previously owned Lightroom must pay full price for the full version of LR6.
The program code for full and install is identical. The only difference is the price you pay.
Version 6 will install alongside your version 5. It won't remove version 5 during the installation process. It's up to you whether you leave version 5 installed or uninstall it.
Lightroom 6 is not an update, but rather an upgrade. It must be purchased. It will not upgrade your Lightroom 5, but will install in addition to an older version. Lightroom 6 will prompt you to allow it to update your catalog that you have been using. You should allow this option. This will create a new copy of your Lightroom 5 catalog for use with Lightroom 6, and will leave your Lightroom 5 installation and catalog intact and usable until you decide to remove it.
I would suggest that you wait a few weeks before making the move to LR6. Look at the list of bugs and issues in this community. I think the list is now as long as a flight from New York to Honolulu.
Thank you all for your advice, I will probably wait a few weeks as suggested, prompt appeared And wasn't sure weather what was actually meant by the upgrade, but sounds like it's an update.
Sent from my iPad