What To Do Before Updating To 10.3
Apple’s move to a new file system APFS. Prior to iOS 10.3, all iOS devices used the popular HFS+ file system. Beginning with iOS 10.3, however, Apple has moved to the Apple File System (APFS).
There are a number of advantages to switching to Apple’s new file system. First: better optimization for NAND flash storage and SSD storage, second: more accurate time stamping, Third: support for stronger encryption.
Here’s the problem, however: once you update your phone to iOS 10.3 and your file system is converted to APFS, you can no longer downgrade without wiping your iPhone and reinstalling an older iOS build. Why is that? Because if something goes wrong and you downgrade to an older build, you’ll no longer be able to recover data because everything will have been converted to APFS.
So, Back Up Before Update to iOS 10.3.
If you haven’t already upgraded to iOS 10.3, be sure to back up all of your data before you do. This can be done either with iCloud (Settings > iCloud > iCloud Backup > Back Up Now) or by backing up using iTunes. It might take a little while, but waiting a few minutes while your iPhone backs up is much less aggravating than losing some or even all of your data.