In counselling you have a option to float and you can change your branch.

In josaa counselling first you have to register and then you will arrange the college and its branches according to your choices. Higher preference should be in top and lower should be in bottom.

Like if you prefer cse in IIT mumbai and then ece in IIT delhi then IIT mumbai should be higher(top) position than IIT delhi.

If in first round you will alloted any college then you can float or freeze or slide.

If you want to select other college with higher preference then you have to do slide.

If you want to choose other branches with higher preference in same college which you have to alloted then you have to select float.

And if you are happy with the alloted option and you don't want to change it then you have to freeze you counselling.

you have to visit nearest NIT or IIT centre.

All the best !