Erase old iPhone 6s+
How can I have an old iPhone erased that hasn’t been used in over 5 years?
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Hello,
If it will start up, do this:
Go to Settings and tap General > Transfer or Reset [device] > Erase All Content and Settings.
If it won't start up, follow section "If you no longer have your iPhone or iPad" in this link:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Good luck.