Issue while modifying and importing the manually content type data I have a scenario like this:

I am using Feeds Importer to import one content type data. I am using GUID field as the unique field. Also, I am exporting these all the nodes of this content type using Views into a .csv file. 

My problem is that users can create nodes using the Feeds Importer and also manually in the Drupal admin console. 

While I export the nodes of the content type, it contains all nodes (manually created nodes and Feeds imported nodes). There will be no GUID for the manually created nodes. 

In this case, how can I resolve the issue of re-import of this data into the Drupal tool?

Is there a way to update the GUID field in the database whenever a new node of this type is created? This will resolve the issue.
Install Universally Unique IDentifier and use it as GUID.it is not required you create GUID field in most of time Nid is good choice  as GUID mapper.

Universally Unique IDentifier Description :       


  This module provides an API for adding universally unique identifiers
  (UUID) to Drupal objects, most notably entities. Features in Drupal 7
  version Automatic UUID generation
  
  UUIDs will be generated for all core entities. An API is provided for
  other modules to enable support for custom entities. UUID API for
  entities, properties and fields
  
  With this unified API you can load entities with entity_uuid_load() so
  that all supported properties and fields are made with UUID
  references. You can also save entities formatted this way with
  entity_uuid_save() (depends on Entity API). Export entities to use as
  default/demo content
  
  The integration with Features module provides the ability to export
  UUID enabled entities with intact dependencies and references to other
  entities. This functionality depends on Deploy module 7.x-2.0-alpha1
  (soon to be released) and is probably the most robust way for
  installation profiles and distributions to provide demo content!