Hey Aspirant ,
After checking the first mock allotment result of JoSAA, the candidates will have the option to alter the choices if the trail/ mock allotment is unsatisfactory. The mock allotment gives a basic idea on which college and branch that a candidate can get based on his/ her rank in the entrance exam.
You can use JEE Main 2020 College Predictor of Careers360 https://engineering.careers360.com/jee-main-college-predictor? .
Good luck with your future endeavors .