how to update a cell value in mysql code example

Example 1: mysql update add to existing value

mysql> UPDATE tableName SET sometotal = sometotal+10 WHERE Id = 4;

Example 2: php mysql update all rows in table new values

$idresult = $conn->query("SELECT id FROM pictures");

while ($row = $idresult->fetch_assoc()){
   mysqli_query($conn, "UPDATE pictures SET UniqueKey = '$UniqueKey' WHERE id = " . $row['id'];
}