Task ID is by default not a mandatory field for tasks.
There are many (1000+) tasks which have blank/null Task IDs (prexternalid).
Is there a way to update the task ids for these tasks having blank task Ids?

Release:
Component: ODXOG

If you are using either MSP or OWB, you could user below workaround. We would recommend to do it in OWB if possible over MSP.

A workaround to have those task ID's populated manually in Microsoft Project and save them back to PPM can be done with the following:
- Use Excel's autocompletion feature to populate a column of cells with consecutive ID's (i.e. like 1, 2, 3, 4, etc.) 
- Copy and paste that column into the project task list in Microsoft Project in the Text1 column (this column will need to be added to your view by selecting it from the drop-down fields) 
- Save the project back to PPM (you'll see that the task ID's are now populated)

XOG is not an option to update as it would create another Task as opposed to updating an existing one.

We don't support doing direct database updates because it may cause adverse scenarios.