change where clause based on row 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: mysql change value

UPDATE [LOW_PRIORITY] [IGNORE] table_name 
SET 
    column_name1 = expr1,
    column_name2 = expr2,
    ...
[WHERE
    condition];