I found this article helpful when researching how do I backup my old iPhone in iTunes before getting a new iPhone, I wanted to make sure I had a good local backup and I did not have enough space locally to accommodate my iPhone backup(s).
These are their steps that worked for me:
Changing the iTunes iOS backup folder on macOS
Launch the macOS Terminal application. This can be found using Finder, under Applications / Utilities / Terminal. Alternatively, it can be launched using Spotlight by pressing ⌘ Cmd + and typing “Terminal” before pressing ⏎ Enter.
Instruct the Terminal to change to the default iTunes backup location, by entering cd ~/Library/Application\ Support/MobileSync and then pressing ⏎ Enter.
Reveal the current backup folder in Finder by entering open . and then pressing ⏎ Enter.
The Finder window that opens will show a folder named “Backup”. This contains any iTunes backups which are already present on the computer. In order to change the backup folder’s location, this folder must be renamed, moved or deleted. As an example, one could rename it to “Backup (Old)”.
Once the “Backup” folder has been renamed, moved or deleted, a symbolic link can be created to the desired backup location. Use the following command ln -s [desired-new-backup-path] ~/Library/Application\ Support/MobileSync/Backup. Once this command has been entered, press ⏎ Enter and the change will be complete.
After restarting the Mac, iTunes will store its backups in the new location.