v5.7.1 is the stand-alone version, not CC. (I think I am correct here!!)
The following steps may get you upgraded, if problematic post back for more 'expert' advice from others.
Backup the catalog,
Sign out of Adobe from the Lr Help menu,
Open the Creative Cloud App. If you do not have it you will need to download, install, and sign-in to Adobe from the preferences menu in the Creative Cloud App.
Click on the [Install] buttons in the CC App to install Lr-CC2015.7 (and Photoshop-CC2017 )
After installing start LR from the CC App by clicking the [OPEN] button.
Lr CC2015.7 will want to upgrade your catalog when you first run it.
Note: If you have a 32bit operating system, you cannot upgrade to Lightroom 6/Lightroom CC 2015 and so you cannot get the dehaze tool.
What exact operating system are you using and what bit, 32 or 64?
If you are still using a 32bit OS even if you subscribe to the CC Photographers plan you can't go any higher then LR 5.7.1.
You first need to install a 64bit operating system to use LR CC 2015/6.