Depends how much you have pledged on Kickstarter (when it happened) or Indiegogo. https://www.indiegogo.com/projects/infinity-battlescape#/
If you have purchased at a Developer Access tier, you can go to https://inovaestudios.com/Battlescape, download, login and get started! Make sure you use the same email address to login here as you did to pledge.
Access for people who have pledged at an Alpha level have a little longer to wait, but it will be soon (June apparently).
There are no further dates scheduled yet.
Also, if you find you have a little extra cash at some point, you can upgrade your pledge here: