iMac High Sierra-Windows 10
If you installed over the main partition, then I would assume you will not be able to go back and boot macOS. You will need to reinstall macOS. Windows should be installed to BIOS boot. If you did this, then you probably will have to erase the entire drive before reinstalling macOS. If you installed Windows to EFI boot, then you will not have to, but you should since the Window Support Software was designed for BIOS booting Windows.