You would change your keys using verification from your designated account recovery person, which by default is steem if you created your account through the steemit website: https://steemit.com/steemit/@steemit3/third-update-to-july-14th-security-announcement-account-recovery-begins
You just need access to a private key that was valid in the last 30 days, and confirmation from the person listed on your account as the recovery account. You can see that your recovery account is steem here: https://steemd.com/@doogee