My iPhone's storage keeps decreasing!

Please check what's taking up the storage and delete it:

Settings → General → Storage & iCloud Usage → STORAGE - Manage Storage

You'll get a list of all the apps and how much storage they're consuming in descending order

Beware that apps such as the so-called Social Media and even mundane ones like Mail cache information for faster response and hence will consume ever increasing amounts of storage space. Check configurations for clearing such cache or delete and reinstall (will erase all associated info).

As for updating I emphatically recommend using the iTunes program on a computer (Mac preferred) to carry it out. Free space requirements on the device will be dramatically decreased, a backup will automatically be done (set it to store locally on the drive, with password, for best results), and the computer will generally have much better downloading and verification capabilities.

Start here:

https://support.apple.com/en-us/HT201656