how do you vote on an item in retrospective

[Claudio Saucedo]

You can try the Agile Retrospectives Add-on for Confluence Cloud. Which has a stage where you Vote for Topics or Issues that team members believe are important to discuss and create action items.

[Claudio Saucedo]

Here is a screenshot where everyone votes on the Issues/Topics.

And then they group them to create Action Items.

[Nic Brough -Adaptavist-]

What do you mean by "item in a retrospective"?  Generally, a retrospective is a talk about how the last sprint went.  There's no need to "vote" on things you discuss, you're just talking about what was delivered, what was not and so-on.  The items are more likely to be "work items", which you handle in an issue tracker.  If it's JIRA, voting is easy - click "vote for this issue"