Hi,

If your PCM aggregate is 66.6% and you're planning to give an improvement exam to meet the 75% eligibility for JEE Advanced, then yes—you can appear for improvement next year. Make sure you improve in at least the required subjects (Physics, Chemistry, and Math) to push your aggregate above 75%. CBSE allows improvement in one or more subjects.

Just ensure you appear only in the subjects you want to improve, and keep checking JEE guidelines for updates.

Hope this helps!