Steps:
Add a Notice for Internal Users at least 1 hour before starting the exam refresh. Here's the page link on how to create notice.
At the update time, go to the exam to refresh, and change the status to 'Draft', so that users are not able to access during the refresh
Go to the Exam Refresh page in the Admin Tools app in Salesforce
Select the exam and certification
Run the exam refresh
If there are not enough questions for a particular section, take a note to adjust later with mappings or new assignments
Once complete, change the status of the exam back to 'Published'.
Change the status of the Notice to 'Draft'. Here's the page link on how to make the notice draft and unpublished.