After you have paid the fees online on the JoSAA portal or through challan you have to report at a physical reporting centre nearby your home for document verification process where you will fill a form to select if your want to freeze float or slide your currently allotted college. 

Once you have completed the physical reporting process for any round after which your choice ( of float freeze or slide) continues and you can implement that online too. 

But for the time being report at the nearest RC as soon as possible and get your documents verified