Code:
$ b=$(echo $b | awk -F, -vOFS=, \$14=$c)
$ echo $b
50,0,0,40,1,0,5000,gold,0,0,0,0,32,11,2,0,10000,0,0,0,0,0,0,0,0,0,BSNL_SMS_Bundle ,null,null,null,null,null,null,null,null,null,0,0,0,0,0,0,0,0,0,0,50,0,0,0,0,0,0,0,0,0,null,null,0,0,405564245,0