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