I am assuming since the value is an array you are updating some type of field that has sub fields.
When you use delete_field() this delete the field and the field key reference for the field and makes the field as if it never existed.
When using update_field() for any field that does not already have a value in the database you must use field keys. You must use the field key for the repeater and you must supply a value that is an array of field_key => value pairs.
See the section on updating via field keys on this page https://www.advancedcustomfields.com/resources/update_field/
