how to change my exam center ..i have applied through online

In my view, its not possible, institute wont entertain such change of exam centres.

Well dear, it seems to be impossible to do that but you can try by personally with ICAI by handwritten application

When i was in PE II, i did quoted my exam centre wrongly. So i made a new application along with annexure stating  OMR sheet no and DD no, to cancel former application and accept latter application, wit attestation of CA. My 2nd application was accepted.

It was manual application and quoted wrongly, Bt in ur case it is Online application. I hope u could hav been provided unique application no, quote tat and make a new application, stating to cancel the earlier.