Nafiseh, could you explain your question a bit more? The "to-do" works like a regular SharePoint task in that you have a status field where you change the value to "Completed" to complete the task. What is it you're wanting to change?
Hi Nafiseh, I'm not sure that you can change the label in a "Assign To-Do Task" action but you can change the labels in a "Assign Flexi Task". So if you don't get another answer, consider using the "Assign Flexi Task".
Eric Rhodes, a consultant with Nintex partner Codesigned, has a great blog post about how to get the 'Complete Task' effect on a to-do task. Check it out: Build a Better Nintex Workflow “Assign To-Do Task” Form | TechRhodes
Unfortunately, the Edit Task Form is not available in the Assign to-do task. It is, however, available to other task actions like Request Review/Approval and the Assign Flexi task. See Create a custom approval form with InfoPath 2010
Alternatively, the easiest way in my estimation, would be to use a Flexi Task and then use Nintex Forms to change the text on the save button. Inside of the configuration dialog of the Flexi Task, simply click on Edit Task Form and then inside of the forms designer, just double click on the save button and rename it. One could also do the same via InfoPath.