Code:
for i in $(seq 10);
do
echo "a,2,c,d,5,6" | awk -v v=$i 'BEGIN {FS=OFS=","} {$2=v$2;$3=v$3;$5=v$5}{print}'
done
Output:
a,12,1c,d,15,6
a,22,2c,d,25,6
a,32,3c,d,35,6
a,42,4c,d,45,6
a,52,5c,d,55,6
a,62,6c,d,65,6
a,72,7c,d,75,6
a,82,8c,d,85,6
a,92,9c,d,95,6
a,102,10c,d,105,6