If I would upload a alpha of my game for everyone to playtest do I need to write a eula for the playtest version?
If it’s publicly available, yes.
Can I edit the eula later (when the game is finished) ?
Yes - usually EULAs permit later amendment/modification
but one last question am I able to sell my game with the same eula wich I created the free playtest with or do I have to create a new one?