How to change field value of a node
1) Use EntityFieldQuery to load a list of entity ID's.

2) Load the full entity using entity_load.

3) Optionally wrap the entity using entity_metadata_wrapper.

4) For each entity, manipulate the field to your desire then save the entity. If you have it in a wrapper use $entity_wrapper_object->save() or if using the entity it's entity_save.

If you want to do this quickly and easily, install the devel module and use the PHP execute page @ example.com/devel/php. Go through each step and ensure you have the correct data to work with at each stage.

Optionally you could use something like VBO to create a view and manipulate the data using a PHP field. This might be easier depending on your experience level.

How to use EFQ