Voting will end on 15 July 2017 at 11:59pm Pacific Daylight Time (2:59am Eastern Daylight Time, 06:59 Greenwich Mean Time, 08:59 in Finland, all on 16 July)
You have to be a member of worldcon to vote. If you are, you should have already received a personalized link to your ballot.
There's some really good stuff up this year, so do be sure to let your voice be heard. The Hugo is the oldest and most prestigious award in SF and fantasy, don't believe anyone who tells you otherwise.