How to update a list field for all the existing items

Hi, 

We had a few choices for "Status" field in a list. Now we are planning to change the choice values. So we will have to update the Status value for all existing list items as well. 

Is there a code which we can use to update all the existing list items with the new value for Status field. 

Like: If status value is "approved", change it to Approved By Manager

If status value is "rejected", change to "Rejected By Manager"

Thanks.