To extend deadlines for appraisals, go to the Appraisals page. You can extend one appraisal's deadline(s) at a time or you can do this for multiple appraisals using the Batch Operations feature.

For a single reviewee, press the Edit button next to their name.

For multiple reviewees, use the Batch Operations > Update Appraisals option.

Select the reviewees whose deadlines you wish to update.

Choose the new self-review deadline, new manager deadline, and press the Next button at the bottom of the screen.

Be sure to press the Update button at the end to register the changes.