Update value inside array update_post_meta

The structure of the array you’ve provided suggests that you’re trying to access an incorrect level of the array. Try to replace:

$t[0]["_give_amount"] = 25.00;
$t[1]["_give_amount"] = 65.00;
$t[2]["_give_amount"] = 100.00;
$t[3]["_give_amount"] = 250.00;
$t[4]["_give_amount"] = 650.00;

with:

$t[0][0]["_give_amount"] = 25.00;
$t[0][1]["_give_amount"] = 65.00;
$t[0][2]["_give_amount"] = 100.00;
$t[0][3]["_give_amount"] = 250.00;
$t[0][4]["_give_amount"] = 650.00;