To answer my own question. You can go to "Entries" tab and choose a game and it will let you give them 1-5 stars. You can edit your answers afterwards, hence you could give review to a game right after having completed it, and then after having completed all the other games, revisit that review page to see if you decide to change your ratings of that game.
But it is still an open question if anyone can vote or only those who participated, i suppose anyway that to vote, you do need itch.io account at minimum.