Deleting personal information from my iPhone

I assume you mean they signed into your phone with their own Apple Account. If that is what you are referring to, that can only happen with your permission since it requires your Password to remove your Apple Account. Now you have 2 options:

• Ask her to remove the account from your phone.
• If you are the owner of the phone and have the proof of purchase documentation, then request an Activation Lock removal. This would only be possible if your device has not been placed in Lost Mode. It also results in all data on the phone being wiped, but you would be able to restore your data as long as you had a backup.

https://al-support.apple.com/#/al/agreement

Refer to How to remove Activation Lock - Apple Support There are only two ways to remove a lock:

#1 If you have the necessary information, you or the signed in owner can do this yourself by simply signing in with the owner account and password information and doing as instructed in the above article. If you use this method and this is a new device to you, follow unlocking by doing as outlined in What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support or What to do before you sell, give away, trade in, or recycle your Mac - Apple Support . If you do not have the required information then you can try method 2 —>

#2 Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Read the requirements and steps for an Activation Lock support request at: https://al-support.apple.com/#/additional-support and links at the bottom right for the type of service requested. Apple says they “may” require "proof of purchase documentation” and that you need “proof of ownership documentation”, but does not detail what currently constitutes proof other than it “must include the product serial number, IMEI or MEID.” User reports on this community and chat with Apple indicate that typically Apple will absolutely require, and ONLY accept, an original receipt issued by Apple or an Apple Authorized Reseller. Presumably this is the only kind Apple can authenticate against fraud since Apple is not involved in third party sales. Apple recommends that you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support

Note that the device cannot have Lost Mode set on it or be a managed device. If your device has been taken over by you having entered a scammer’s Account then the scammer has likely set Lost Mode and the device is lost to you.

You do NOT have to be the “original” owner. In the case of removing the lock with the correct information, it can be anybody who knows the currently signed in account information. In the case of asking Apple to unlock it, as long as you have the original Apple receipt (which may well not have any personally identifying information on it) you are okay. Apple confirmed this in a conversation.

If Apple resets the device this will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.

If you cannot get the device unlocked then read this section from the very bottom of Apple Trade In - Apple :

"Does Apple offer recycling?

Yes. Apple Trade In lets you recycle any Apple device (including devices from Apple-owned brands) at any Apple Store and on Apple for free."

If your data are mixed with the other person's data you may need to do as instructed here --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Activation Lock?

Whoever that someone is here will need to remove Activation Lock.

Otherwise, and only if the phone is also not marked as lost — the usual ransom scams here will mark as lost, blocking all means of rescinding the ownership transfer — then present the original purchase receipt from Apple or an authorized reseller with an Activation Lock request to Apple.

If the holder of the Apple Account holding Activation Lock is unavailable or unwilling or unable to relinquish iPhone ownership, and particularly if the phone is in lost mode, it is now and always will be a brick.

Activation Lock is intended as a theft deterrent, and is intentionally difficult to bypass. We can’t help, and — absent convincing proof, or in lost mode — Apple won’t help.