Voters cast their ballots during early voting in North Carolina on October 20, 2016. Thomson Reuters

This year's contentious presidential election has drawn early voters out in waves, with both Democratic nominee Hillary Clinton and Republican nominee Donald Trump calling on supporters to cast their ballots before Election Day.

At least 37 million people had already voted, either in person or by mail, by Friday, according to the US Elections Project.

But recent developments in the campaign may have some voters wishing they could change their mind.

Three states — Michigan, Wisconsin, and Minnesota — allow voters to switch their votes before Election Day. Despite news reports, Pennsylvania does not allow absentee voters to change votes. In Connecticut, it is up to the local town office, a State Department official told Business Insider.

How to change your vote

Changing your vote can be quite the headache for election officials, since they have to track down your ballot and re-file the paperwork.

In Michigan, absentee voters can change their ballots if they haven't been processed and tabulated yet, according to an official at the state's Bureau of Elections.

If you want to change your absentee vote there, contact your city or town clerk's office. The best chance to change your vote will be before 4 p.m. on Monday (since Election Day is often too busy or too late). Offices are open Saturday until 2 p.m.

Wisconsin residents who voted early in person can recast their ballot so long as they do so by Saturday, November 5, an election official told Business Insider. The deadline for absentee voters who mailed their votes in to change their choice has past, he said.

Minnesota also allows absentee voters to change their minds, but the deadline to do so was one week before Election Day.

'A big mistake'

The call to change votes began last week after news broke that the FBI was looking into more emails from Clinton's private email server. Trump quickly encouraged voters to switch their choice.

"Now that you see that Hillary was a big mistake," Trump tweeted on November 2, "change your vote to MAKE AMERICA GREAT AGAIN!"

The candidate was slightly off on the number of states that will allow the unusual practice (he was likely referencing a Fox News report that was later corrected, or perhaps a CNN story that erroneously listed seven states).