If you need to revote on an agenda item, use the agenda item dropdown to navigate back to the item, and you will be shown a dialog box that asks you what you wish to do:
If you want to make a copy of the original agenda item, choose the Copy option. This will create a duplicate of the item and place it after the original item in the agenda.
If you want to revote on the item, choose the Revote option, but be aware that this option will results in your overwriting previous results on this item.
If you selected an agenda item that has already been processed by mistake, choose the Cancel option, which will return you to your previous agenda item & stage.