Updating value of array

Updating value of array

how do I update the value in itemprices if iteminfo equals to certain
subject?
User(
[name] => xxxx
[phone] => xxxxx
[email]xxxxx
[itemprices] => Array ( [0] => 1.00 [1] => 1.00 )
[iteminfo] => Array ( [0] => Chemistry [1] => Biology )
)