The Computed Field module only updates its computed values on saves or on load (if no computed value is computed yet). This is not optimal when using computed values in lists etc., or if you should wish to change the logic in the computed field.
The solution
If you wish to avoid re-saving all the entities/nodes using the computing field, you can use this tool to re-compute all the values again.
The computed field tools module offers a way to re-compute the computed fields of existing entities/nodes. It does so through the Batch API.
When the batch is running it does not save the entire entity/node, but it only saves the computed field. This make it a fast way of updating the content.
Features
Fast processing of all computed fields.
Select specific computed_field to process.
Select content types to process for selected computed field.
It processes ALL entities/nodes. Also the ones with no value computed yet.
Since we don't do an entity_save() we don't clear the cache on every(!) node processing and no search index updates are triggered, so there is a huge performance advantage here on high traffic sites over the VBO way.