Picklist fields contain a list of one or more items from which a user chooses a single item. They display as drop-down lists in the Apto user interface. One of the items can be configured as the default item.
Multi-select picklist fields contain a list of one or more items from which a user can choose multiple items. One of the items can be configured as the default item. Selections are maintained as a string containing a series of attributes delimited by semicolons. For example, a query might return the values of a multivalue picklist as "first value; second value; third value".
Navigate to the appropriate object and open the Quick Access Toolbar
Click View Fields
Locate the field you're attempting to modify and click on the Field Label. Do not click edit to the left of the field label.
Scroll down to locate the Picklist Values section
If you would like to modify an existing value
Click Edit next to the picklist value you'd like to modify.
Add the new value and click Save.
If you would like to create a new value(s)
Select the New button and add the value(s)
You can enter multiple values and separate them by returning to the next line down.
If your organization uses record types for the object you're manipulating be sure to mark the record types this value should be available for.
If you can see the picklist values on the fields, but they are not available when you are working with a record it is because they have not been added for that records record type.