According to some new research, the hackers may have access to some of the data they claim, although it may not mean all the 300 million accounts are likely to be compromised. This is usually because people have reset their passwords, or e-mails since the apparent theft of the data.

Two of the people we spoke to confirmed that someone had tried to reset their iCloud accounts in the past day. One of the people said that they had received login notifications on Twitter, which used the same iCloud email address and password. This seems fitting with the hackers' apparent desires to reset accounts as they claim. Apple iCloud ransom demands: The facts you need to know [zdnet.com]

If you have two-factor authentication it should be ok.

The advise must be to make sure you have two factor authentication, and then to reset your password as soon as possible, but do watch out for attempts to reset your password that you did not initiate.
Here's a link to the Apple password reset.
[iforgot.apple.com...]