Hello, Priya
I understand you would like to know a way in which you could reduce the Size of your CloudSQL instance, consequently, reducing your Billing Charges for the present storage. Sadly, at the moment, there is no way to actually achieve this[1]. There is a feature request[2] available for this implementation and the CloudSQL team are reviewing the possibility, how it fits into the Road-map of CloudSQL. At the moment, however, this is not possible.
A workaround, as addressed in this Stackoverflow link[3] would be to create an SQL dump file[4] of your CloudSQL Database, create a new Cloud SQL instance with the desired capacity, then import the SQL dump file so that all your data is stored in the new instance.