Hello Aspirant. Yes you can surely appear for JEE MAINS on April. NTA will be taking best of the two percentile if you are appearing for both the attempts. Suppose you got 83 percentile in January exam and you get 97 percentile in the April exam then 97 percentile will be considered while preparing the Overall merit list.

You can check the below link to know more about percentile and normalisation of marks.

https://engineering.careers360.com/articles/jee-main-normalisation-process-how-scores-are-calculated

Hope this helps. 

Good luck!