You can mark any question for review and come to it later.
You can change answer of any question at any point of time during the exam. This is I think one of the biggest advantages of JEE Main’s online exam format.
If you have opted for the online JEE Main 2015, I would suggest you take a mock test at CBSE JEE website when it is available. After form filling or maybe a few weeks before the exam, CBSE will launch officially a mock test.