Updata field separated by comma

Hi
I've field contain data from an array separated by "," . like data1,data2,data3,data4 .

I inserted them by and working OK .

foreach($array as $b)$result .= $b . ',';
return $result;
Also when i show them i explode them 1st & i put in smarty HTML page where i want to these data like $array[0] , $array[1] , $array[2] . gave me data1 data2 exactly as i want .

But i can't update the data . I want to know how can i update or replace the data .
Please help