How do I deactivate old iPhone ready to sell

Since you say you wish to sell this device you may be interested in this document -->

What to do before selling or giving away your iPhone, iPad, or iPod touch [or if you received one that had not been erased] - "Use these steps to remove your personal information from a device, even if you don't have it anymore." Click here --> https://support.apple.com/HT201351

Here’s a more complete answer than the link above your post.

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

See instructions in: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Use the link above your name on this page.