The default location is on the primary drive at:
C:\Users\YourUserName\AppData\Roaming\Apple Computer\MobileSync

Let’s say you want to change it to:
G:\Backups\YourUserName\iTunesBackups

Run these commands in PowerShell:

c:
cd “C:\Users\YourUserName\AppData\Roaming\Apple Computer\MobileSync”
mklink /d Backup “G:\Backups\YourUserName\iTunesBackups”