To start go to Setup (upper right) -> Customization -> Modules
- Select the module from the drop down list
- In the display of fields available for that module click the gear wheel next to the field whose value you want to change and click Edit properties
- ADD the new value, but DO NOT remove the old value yet!
- Add new value by hovering over the value under which the new value will appear in the list and click the "+" sign on the right. Type in the new value.
- Click Modify to save the new value as an option in the list
- Again, click the gear wheel next to the same field and this time choose "Replace Values"
- Choose the Existing value in the top list
- Choose the new value in the bottom list
- Click Replace.