I had the need to change a reminder date of a workflow task in order to test a cron job sending out email reminders.
Using the workflow console (http://localhost:8080/alfresco/faces/jsp/admin/workflow-console.jsp)
First query to find running workflows.
show workflows all
Identify the workflow you are interested in.
Using the output of the previous commands we drill down to what we need:
show paths activiti$710
desc path activiti$788
Update our reminider date using ISO8601 date format:
update task activiti$791 lysewf:reminderDate=2014-10-06T12:00:00+01:00
Check that the date has changed:
desc task activiti$791