If you mean, for example, skip question 10 and come back to it [u]after you've moved on[/u] to question 11, NO.
If you mean pick an answer or two, out of say the four possible A through D, and then change your answer, before moving on to the next numbered question, then YES.
Once you have moved on to the next question, that's it.
One thing you can't do on the real test, that you can on the practice tests, is check all four A through D, and then un-check the the ones you figure are wrong.
The last is a pet peeve of mine, but it doesn't seem CWNP has any plans to change that.