Regarding credit note

My client was deceased in 2017. His RC was closed before GST. His son ruin the same business venture in other name and it's registered under GST.
Now some Stock in 2017 which were purchased in old RC was returned due to scrap in the name of New RC taken @ 18% of GST.
How we claim credit note and how to make transaction in our books of accounts and GST... would u guide me....