Cancellation of GST registration of composition scheme dealer

I want know online procedures of cancellation of GST registration of composition dealer. I have doubt that whether dealer is required to pay tax on stock held on cancellation date? Second if his current year(2019-20) turnover is 42 lacs then can he opt for surrender of GST registration on basis of his expected turnover will be less then 40lacs in FY 2020-21.plz solve my doubt if possible quote sections and rules also