I am not certain what is meant by "instance". I think your question is asking how to delete a field from a Case Type or Data Type in App Studio.
The "data model" for each Case Type and Data Type will list all of the relevant fields for the object. Each row defines and individual Field, with a gear icon to configure the field and trash can icon to delete the Field. This will take the appropriate action to remove the field from the object.
- Field is defined in the class for the object, created in the highest ruleset version of the ruleset, and it is unlocked: The rule instance will be deleted
- Field is defined in the class for the object, rule exists in lower ruleset version: A withdrawn rule will be created
- Field is defined in another class: An inactive relevant record for the field will be create to hide the field from the object.
Using the trash can, you should be able to remove the single record field from the object and then create a new multi-record field.