If the applicant is Active, click the check mark to the right of the status change you wish to undo. The check mark will then change back to an arrow signifying the step as incomplete. 

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If the applicant is Inactive, make sure your filter options are set to include Inactive Applicants (select this option under 'More Filters' at the top right-hand corner). Locate the applicant, then click Make Active to move the applicant back to active and then follow the above steps to undo a hiring process step.

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