First of all if you purchased a used Mac, then the third party software on that Mac may not legally belong to you.
Second, whenever you purchase a used/preowned computer you should always perform a clean install of the OS by first erasing the whole physical drive before re-installing the OS so that you have a known good factory clean OS. You never know what condition an OS was left as and what nasty surprises may be lurking there from the previous owner. Here is an Apple article with instructions that the previous owner should have followed before selling the Mac:
What to do before you sell, give away, or trade in your Mac - Apple Support
Even if the previous owner did all these steps, I would still highly recommend performing the clean install yourself to be 100% sure the Mac has a good clean OS on it (step#7). By performing the clean install yourself you may find out whether the Mac has a firmware lock or whether the Mac is still being managed by a previous owner, both of which are extremely bad things requiring the Mac to be returned for a full refund since Apple will not be able to remove these items since you won't be able to prove you are the legal owner.
You can try booting into Recovery Mode and using Disk Utility to attempt to convert HFS+ to APFS. There should be an option on one of Disk Utility's menus to convert the file system. While it should not hurt the data on the drive you should make sure to have a good backup just in case something goes wrong.