How do I update multiple employees' details in bulk?

You can update some employee details in bulk on the Employees screen by checking the boxes for all employees you want to update, then selecting Modify from the Bulk actions dropdown:

A pane with all bulk-editable properties appears on the right:

Set the properties you want to change, then click Bulk update at the bottom of the pane. All ticked employees will have their details updated to the settings you selected.