This reference explains the Custom Object: Update Record task.

This task updates data fields of a custom object record.

When creating the task, you need to select the custom object, specify the ID of the object, and provide the new values of the data fields that you want to update.

If you have created a custom object definition or a field definition of a custom object but it is not displayed in the drop-down list, refresh the Workflow cache. To learn about how to refresh the cache of Workflow, see Workflow maintenance settings.

To learn more about custom object definitions and records, see custom object definitions and custom object records.