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You can change your keys by going to https://steemworld.org/@throwbackthurs and there scrolling down to "Change Password", where you can change your password.
You can probably do this in at least a dozen other ways, but steemworld is probably the one I'd use.
But as @enforcer48 already said, make sure that they don't have your posting authority either, since then your new password won't stop them from posting in your name.

I've searched a bit, and, well, it's tough to find.. One option that is mentioned everywhere is, if you change your main password on the steem-wallet site, all keys will be re-generated from that new password. Another option is to use cli-wallet. It is supposed to have commands that allow you to change any of the keys selectively. It's mentioned at least here: https://steemit.com/steemhelp/@pfunk/a-learner-s-guide-to-using-steem-s-cliwallet-part-1 in the description of "part 2" that sadly, was never published.

Also - you may find it useful - https://steemit.com/steem/@luegenbaron/how-to-revoke-posting-authority-and-change-master-key

Make sure you didn't give them your posting authority.

If you have, it doesn't matter if you have changed your password. See @inertia's latest comment.

https://steempeak.com/security/@inertia/pzmv20

great, thank you

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