How to update / Modify existing Task (Activity) records via Import

Hi,

Request a clarification, regarding the below scenario.

1. We have around 1800 activity / tasks records (in our production environment),
2. Now, we want to update values for a new custom field for these tasks records
3. I have Row id's for these activity / task records
4. But, when I try to update these fields via Import
5. In the Import Wizard for Task,
 
- In step 1 - We don't see, the method Import should use to uniquely identify matching records (i.e. Row ID option is not available here)
- And, also the Overwrite Existing Records option is not available here, when we try to import a Task.