Sort order of input values

Hello!
We are using UPC to display pictures through searching as well as 5-6 custom fields. The values for the custom fields are displayed via dropdown. I would like to have the values sorted alphabetically when the user opens the dropdown.
I cannot find any option for sorting the custom field values, they are more or less ‘random’. Defining input values did not help.
Any remedy to this? Where can I set the sort order for the values when displayed, for example alphabetical?
BR
Max