Final answer:
Ana should update the existing User Story with the new profile access information for the ExternalId__c field and then adjust the Salesforce profile settings to provide the necessary field access. This maintains a clear audit trail and ensures changes are included in the deployment process.
Step-by-step explanation:
Best Practice for Updating Field Access in Salesforce
When Ana realizes she has forgotten to give the relevant profile access to the ExternalId__c field, the best course of action would be to update the existing User Story with the new field access information. This update will ensure that all changes related to the task are tracked under the same work item, maintaining a clear audit trail of the modifications done in the development process. After updating the User Story, Ana should then proceed to adjust the profile settings within the Salesforce environment to provide the necessary field access; this may include updating field-level security settings for the profiles that need this access. Once the access has been granted and the User Story updated, she can then commit these changes, ensuring they are included in the upcoming testing, promote, and deploy phases.