For updating the different columns, you had to use the code like this as below
DT_TextFile.AsEnumerable().ToList().ForEach(Sub(row) row("FPP")= CDbl(row("FPP").ToString)/Cdbl(row("Case").ToString))
DT_TextFile.AsEnumerable().ToList().ForEach(Sub(row) row("APSUM")= CDbl(row("APSUM").ToString)/Cdbl(row("Case").ToString))
Try this approach
Thanks & Regards,
Nived N