EF Code First has a neat method to merge in the values of a DTO into an mapped entity.

context.Entry(entity).CurrentValues.SetValues(dataTransferObject);

The DTO will generally have a primary key property, and you can use that to determine if it is a new record or a modification. Here's a method that does that:
public static T Merge<T>(this DbContext context, object dataTransferObject)
             where T : class
        {
            if (context == null) throw new ArgumentNullException("context");
            if (dataTransferObject == null) throw new ArgumentNullException("dataTransferObject");
 
            var property = FindPrimaryKeyProperty<T>(context);
            //find the id property of the dto
            var idProperty = dataTransferObject.GetType().GetProperty(property.Name);
            if (idProperty == null)
                throw new InvalidOperationException("Cannot find an id on the dataTransferObject");
            var id = idProperty.GetValue(dataTransferObject, null);
            //has the id been set (existing item) or not (transient)?
            var propertyType = property.PropertyType;
            var transientValue = propertyType.IsValueType ?
                Activator.CreateInstance(propertyType) : null;
            var isTransient = Equals(id, transientValue);
            T entity;
            if (isTransient)
            {
                //it's transient, just create a dummy
                entity = CreateEntity<T>(id, property);
                //if DatabaseGeneratedOption(DatabaseGeneratedOption.None) and no id, this errors
                context.Set<T>().Attach(entity);
            }
            else
            {
                //try to load from identity map or database
                entity = context.Set<T>().Find(id);
                if (entity == null)
                {
                    //could not find entity, assume assigned primary key
                    entity = CreateEntity<T>(id, property);
                    context.Set<T>().Add(entity);
                }
            }
            //copy the values from DTO onto the entry
            context.Entry(entity).CurrentValues.SetValues(dataTransferObject);
            return entity;
        }
 
 
        private static PropertyInfo FindPrimaryKeyProperty<T>(IObjectContextAdapter context)
            where T : class
        {
            //find the primary key
            var objectContext = context.ObjectContext;
            //this will error if it's not a mapped entity
            var objectSet = objectContext.CreateObjectSet<T>();
            var elementType = objectSet.EntitySet.ElementType;
            var pk = elementType.KeyMembers.First();
            //look it up on the entity
            var propertyInfo = typeof(T).GetProperty(pk.Name);
            return propertyInfo;
        }
 
        private static T CreateEntity<T>(object id, PropertyInfo property)
            where T : class
        {
            // consider IoC here
            var entity = (T)Activator.CreateInstance(typeof(T));
            //set the value of the primary key (may error if wrong type)
            property.SetValue(entity, id, null);
            return entity;
        }