Yes you have to do the correction in your JEE mains form because the mistake that you have done is a big one.

The correction in particulars can be done only if the candidates have paid the fee and completed all the steps of online application form of JEE (Main) –2018. The candidates can modify all the particulars including city of examination, Paper - 2 to Paper -1 and vice versa or select both papers of his/her application form during the facility of correction in January, 2018. The balance fee, if any due to correction in particulars, will have to be paid by the candidates during that period. However change of mode of examination from computer based to pen paper based examination is not allowed.