How to bulk update select list fields?
You have multiple options:


You can use code that iterates over all nodes and changes the value of the field (you will need some module to execute the code, for example devel).
Other option is to use the Views Bulk Operations module, build a view which shows a list of nodes of that type, and add a views bulk operations field which has the option to edit a node, and this will allow you to select all the nodes in a view and modify a field with the same value.