You can only re-vote from the initial vote or from the Matrix graph (heat map). You cannot re-vote from the List Editor or a scatter plot.
During a vote press R to clear votes so far and ask everyone to vote again.
When you have just voted on a risk and are reviewing the result with the distribution chart showing, press R to restart the vote.
On the Matrix graph right click on the item on your heat map, or right click on the item in the list pop up and select revote on or revote from, then choose the assessment. "Revote on" will open the vote for just the assessment you choose. "Revote from" will open the vote of the assessment you choose and then open the votes for the subsequent assessments.