How can I do a factory reset my Iphone7

See the following Apple Support article for information on completely resetting an iPhone: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Open Settings ➡️ General ➡️ Transfer or Reset iPhone ➡️ Erase All Contents & Settings:

This action will delete all data from your iPhone and return it back to factory settings.

Axel F.