How can I change the executor of a task in the process template?

Go to Manage Processes and first unpublish the template you want to edit by clicking the "Options" button on the right side of the relevant process.

Once that's done, enter the process and select the task you want to modify. Then, click "Options → Edit" and change the executor.

After making the changes, you need to re-publish the process, and it will be ready to use.

Note: This change will not update already started processes but will apply from the first process you