Using the built-in tool Disk Utility, you can convert APFS to OSX Extended or macOS Extended (Journaled). Find Disk utility through Spotlight search, open it, select the drive in demand, and then click Erase to convert the format. On the box next to Format choose OSX Extended/macOS Extended (Journaled) and click Erase. If you want to act on the startup disk - Macintosh HD, you need to boot into Recovery mode.