If you want to take refund then you have to file Form 501 (Maharashtra Sales tax), but if you want to transfer credit of VAT in GST then you have to file Form TRANS 1 before 31/10/17.
Follow below steps: Login to mahavat site -- E services -- E refund/ E returns only VAT/CST from FY 2016-17 -- E refunds -- fill up the asked details -- upload file.