You can change your APFS-formatted drive to Mac OS X Extended file format on your Mac with the native toolset -- Disk Utility.
But before converting the file system, let's work with your data on your external hard drive first. File system conversion of the drive erases all files and data on it. Therefore, you must back up all the data in advance.
You're highly recommended to apply iBoysoft DiskGeeker, which can clone your whole drive with its Disk Clone feature, remarkably shortening your backup time with a stable and fast data transferring speed.
Now, let's see how to clone your APFS drive with the help of iBoysoft DiskGeeker:
Step 1: Connect the disks you need to use to your Mac and launch iBoysoft DiskGeeker.
Step 2: Select the source disk (the entire disk you want to back up files from) at the left sidebar of the iBoysoft DiskGeeker window.
Step 3: Click "Clone" from the right toolbar.
Step 4:Choose a destination drive from the pop-up list and click startClone.
After you deal with the data on your drive, you can now format the APFS drive to Mac OS X Extended.
Step 1: Connect the external hard drive to your Mac.
Step 3: Tap the View button and choose Show All Devices.
Step 4: Find your external drive on the left sidebar, the drive information displayed on the right side tells you it is an APFS volume/partition.
Step 5: Control-click the APFS external disk and choose the Erase option on the menu.
Step 6: Fill in the disk name, choose Mac OS X Extended in Format, and click Erase button to begin the conversion.
Step 7: When it completes, you can use the Mac OS X Extended external drive now.