Hello! 

The jee main April 2019 exam will be held between April 7 to 12. Paper 1 of jee main 2019 will be held in cbt mode only on April 8 9 10 and 12. 

While paper 2 will be held partially in cbt and offline mode on April 7.

Candidates can appear for either one exam or both the sessions. In case they appear for both the better of two scores is considered.

https://engineering.careers360.com/articles/jee-main-important-dates

Wish you all the best! 

Unfortunately not many exams offer the choice of changing dates. In case you are talking about JEE main, then it is important for you to know that you cannot alter the dates. Though you can change the test mode form paper pen to online or vice versa but that too cannot be done once the dates for corrections and changes have ended, which, in my view, already has. 

In case there is a scope for you to change the other exam date you can proceed with the plan otherwise , unfortunate as it may sound, you will have to pick between one .