RetopoFlow uses the Add-on Updater module to allow updating from right within Blender!
If you have version 1.3.0+ installed then simply go to the User Preferences > Add-ons > RetopoFlow and click Check now for RetopoFlow update, then Update Now.
Alternatively you can update by first removing previous versions of RetopoFlow and then installing the new version via User Preferences > Add-ons > Install from File. From the file window then choose the downloaded .zip to install.
You sure do! v2.0 is a free upgrade for all prior 1.x customers.
We are actively working on the Blender 2.8 version of RetopoFlow and hope to have it ready very soon; it's been a massive project that requires most of the code-base to be rewritten.
The 2.8 version of RetopoFlow will be a free upgrade to all customers.
Yes! RetopoFlow works on any system which Blender supports, including Mac OS, Linux, and Windows.