Heyo! Unity engine is free to use (the standard edition, which has almost all the benefits that Pro version offers) so you're *free* to get it from their website whenever. :D
You'd first have to get yourself UnityHub and through UnityHub you can download the latest version(s) you plan on using. After that is done, you create an empty project, select if you want it to be a 2D or 3D project and violá. Good luck.