Download & Install
Flow is a framework targeted at being used with your own, Composer-managed projects. As such, some knowledge on using Composer, setting up a webserver and working with a version control system are not only useful but to some degree expected.
The basic installation of Flow is described in the Quickstart tutorial, and the Definitive guide has more details on the requirements and in depth installation and configuration steps.
Archives of releases (not recommended) can be found at the Github releases page, change logs are linked from there as well. The change logs as well as release notes are contained in the documentation.