When the form only contains employee fields (no manager input is required), then resetting the form is as simple as clicking 'Reset Form' instead of approving the form.
When the form contains only employee fields
On the Tasks page, click the name of the task, and click 'Reset Form' instead of approving the form.
When a form contains both employee and manager fields
When a form contains both employee and manager fields, you can reset it after the employee completes their half (because the employee has made a mistake) or after the manager completes their half (if the manager made the mistake).
If the employee made a mistake
On the Tasks page, click the name of the task, scroll to the bottom, and click 'Reset Form.'
If the manager made a mistake
Locate the employee on the Onboarding page. Click their name, then scroll to the completed task that must be reset. Click on the name of the task and then click the 'Reset Form' link.
When you do this, the form will be completely reset, and both the employee and the manager will have to fill out their respective parts of the form again.