a lot of states seem to be compromised by easy to mess with voter websites

https://techscience.org/a/2017090601/

https://www.mcall.com/news/watchdog/mc-voter-registration-fraud-watchdog-20161102-column.html

https://katu.com/news/local/some-oregon-voters-claim-their-party-affiliation-changed-ahead-of-primary

https://www.oregonlive.com/news/2020/05/twitter-suspends-account-of-oregon-group-that-claims-voters-party-affiliations-were-changed-without-consent.html

https://www.oregonlive.com/politics/2020/05/no-elections-officials-didnt-change-oregon-voters-party-id-without-permission-they-say.html

https://www.eastoregonian.com/news/local/elections-officials-dispute-claims-that-voters-registrations-were-changed/article_20cd6d72-a140-11ea-bcad-abbb65c210ee.html

So it looks like you can request a new ballot which will invalidate the one you may have already mailed. You may also be able to change peoples voter registrations. And if they are absentee voters you can print off a new ballot and fax it in or drop it off and change their vote.

So its not quite as easy as lol go online and change vote. But theres obviously room for massive fuckery and this is not secure at all