How to change or update select list of field which is already filled with values
That's quite a daunting task - in order to keep consistency of existing values already stored in your database you can either change all of the nodes containing the value you want to change or add a new option to the select-field.

Changing a lot of options at once could be done by a views-bulk-operation and the operator Change value (not sure, if it's called this way - but it exists ;)).

If i run in such a situation i often write an update_hook to change all values of the field in the db (do not forget to change revision table also) and change the select values afterwards.