Default Boot Is to Windows not High Sierra
Looking at How to switch between Windows and macOS in macOS High Sierra - Apple Support my Win10 partition is on the left side and my High Sierra (APFS) partition is on the right -- while in their graphic it's the other way around. As a result when I restart from Win10 or from High Sierra (or do a shutdown, and start up) it defaults to Win10 every time.
My solution is to hold the Option key when I restart; then it defaults to High Sierra. But is there any way to make High Sierra the default without a complete reinstall, which I'd rather not undertake?
Phil
MacBook Air (13-inch Mid 2013), macOS High Sierra (10.13.1)