Update a field on a node after hitting save I am working on a Drupal 7 custom module and would like to update a field on a node after hitting save.  Based on some post that I have seen this should be possible. 

As it is now I have the $node object is a parameter to the node_submit hook after hitting save.

However, I am not able to update my field on this node object.  Any pointers or examples of how to do this would be greatly appreciated.

Thanks, 
Derek
Depending on your exact usage, you'd want to use either

1) hook_node_presave - Called on any node being saved (inserted or updated). see here

2) Attach a submit handler to the form: Allows to change the form's values before the submit is completed.