dear aspirant,

Say you have selected float(by reporting to the centre) in the first round.Now after subsequent round(say round 2) you get a higher preference(15th) in this round. Now if you wish to select this seat(15th option), i.e. you don't want any other higher preferences you had filled in earlier, you have to report to the centre and change the option from float to freeze. This is done by submitting a form stating you wish to change.

hope it helped.