You can sum up all the remittances received in your bank account from Upwork during the year and add the amount receivable from Upwork as on March 31 2017 to ascertain the total turnover for the year ending 31st March 2017.
If you are getting professional fees from your clients from other sources (like Paypal or direct transfers to your bank account), you will have to add that also to ascertain the turnover for the year.
Also, just to keep you posted, all the freelancers providing services to clients outside state, needs to get themselves registered in GST, from July 01 2017 onwards.
If you have any other query with regards, please feel free to contact.
CA. Ankit Tulsyan