How to Edit Multiple Customers??

You can add multiple customers using File->Import->Contacts.  There is no provision for editing multiple customers on the Mac other than one at a time. 

You can add multiple customers with QuickBooks->Import->Contacts.  You probably want the "Paste from spreadsheet or enter manually" option.

There is no provision in Quickbooks Mac 2019 for editing multiple existing contacts other than doing it individually (there is a provision for editing 1099 information for vendors en masse)