How do I buy more storage ??

I want you to realize and know one important thing and that is that there is NOTHING that you can do to increase the physical storage capacity of your iPhone. Your iPhone's storage capacity will forever remain the same as the day you bought it.

You can purchase ➔ iCloud Storage from Apple where you create and save your iPhone backups and synch (Not Store) your Messages, Photos, Contacts, Calendars, Notes and Reminders. Note that iCloud Storage is NOT an extension of storage capacity of your iPhone. Just wanted you to know some facts. Thanks.

Axel F.