Best practices to map DTO to Entity

This question could be subjective, but I'm hoping to gain some insights into how to properly map DTO back to Entity.

From what I've read and implemented, DTO is the object that hold a subset of value from a Data model, in most cases these are immutable objects.

What about the case where I need to pass either new value or changes back to the database?

Should I work with the data model/actual entity in my DAL?

Or should I create a DTO that can be passed from the presentation layer to the business layer then convert it to an entity, then be updated in the DB via an ORM call. Is this writing too much code? I'm assuming that this is needed if the presentation layer has no concept of the data model. If we are going with this approach, should I fetch the object again at the BLL layer before committing the change?