iCSV=Import["test.csv"]
->{{"C1","C2","C3","C4","C5"},{1,5,9,13,17},{2,6,10,14,18},{3,7,11,15,19},{4,8,12,16,20}}
iCSV[[1, 1]] = Column0;
iCSV[[2, 2]] = 100;
iCSV[[3, 3]] = 200;
iCSV[[4, 4]] = 300;
iCSV[[5, 5]] = 400;
iCSV[[2, 3]] = 60;
Export["test.csv",iCSV];