Increment

UPDATE  table
SET     field = field + 1
WHERE   id = $number

Decrement but don’t go below a value

GREATEST allows you to specify that the greater of multiple values should used. So in this example if the value drops below zero, zero will be greater used:

UPDATE  table
SET     field = GREATEST(0, field - 1)
WHERE   id = $number

Increment but don’t go above a value

LEAST allows you to specify that the lower of multiple values should used. So in this example if the value goes above below 9999, 9999 will be used:

UPDATE  table
SET     field = LEAST(9999, field - 1)
WHERE   id = $number