Hi again Ramesh,
Apologies, this document does show how to change your size, but I didn't click that, without support, you cannot go over 250 GBs for your cloudSQL instance, as explained
here.
There is a reason as to why we limit customers without support, and it's simply because restoring a backup of more than 250 GB can take a long time. We have this 250 GB limit to make sure that, if something goes wrong, there is a recourse for the customer to reach someone in support.
So, while the first doc I linked explains how to change your instance size, you will require support to enable that increased limit for you.
Cheers!