After your vote has been submitted you have the opportunity to change your vote if your vote was done as part of a parallel review and all other reviewers have not cast their vote.
Once all reviewers in the step have submitted their votes, you will no longer be able to change your vote.
To change your vote:
Click on Change vote next to your vote status in the bottom right corner of the Viewer
Update your vote selection from the dropdown
Click Vote and enter your PIN