How to rebuild an Exchange server 2016 Search Index?

open cli and input this command:
Get-MailboxDatabaseCopyStatus *|sortname|Selectname,status,contentindexstate
stop these services:
Stop-Service MSExchangeFastSearch
Stop-Service HostControllerService
locate the path of the db :
Get-MailboxDatabaseDB01\SRVCOM |selectEdbFilePath 
delete  the GUID folder :
now restart the services
start-Service MSExchangeFastSearch
start-Service HostControllerService
and issue this command Get-MailboxDatabaseCopyStatus * | sort name | Select name,status,contentindexstate
As you see, the indexing process is restarted and we need to waiting that switch to healthy
That’s it!