Reversal of GST Paid between the Earlier Contract and Revised Contract

Dear Sir, 

I am a service provider of Consultancy Services. I have taken assignment work wherein the Service receiver has agreed for Total Contract of Rs.15 Lakhs on 07/03/2019 and accordingly given the appointment letter to me. In between of 2022, it was agreed to reduced the Total Contract to Rs.12 Lakhs. I have paid my GST collected amount based on the Invoice raised on Monthly basis. 50% work has been completed.

1. So, how can we reconcile billing between the Earlier Contract and Revised Contract.

2. Can we revised the GST paid on earlier Invoices. 

Kindly suggest.