How to remove someone iCloud account and create another account

It is not possible.

If you have bought 2nd hand iPhone, you must contact the seller (hopefully it is the previous owner), otherwise you have been scammed.

Each Apple device is linked/associated with an Apple ID of the person who did the setup.

Activation Lock prevents anyone who is not the owner from using this iPhone.

To unlock the Phone, you need the Apple ID and password of the previous owner, that were used during set up.

So you have to contact the previous owner.

The previous owner has to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch -> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Then the previous owner has to remove the iPhone from his account, see this Apple article:

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to find where you're signed in - Apple Support

Then you can sign in with your own Apple ID.

See the following Apple articles:

Activation Lock for iPhone, iPad, and iPod touch

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

How to remove Activation Lock -> How to remove Activation Lock – Apple Support (UK)

• If you need help removing Activation Lock and have proof of purchase documentation, you can start an Activation Lock support request.

👉 Read this before you buy a 2nd hand iPhone.

You can also contact Apple support for help, -> Get Support (apple.com)

You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.

If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.

Once Apple receive your proof of ownership, Apple will further review your request and provide an update.

See also -> Keep your receipt after you buy an Apple device

Keep your receipt after you buy an Apple device – Apple Support (UK)

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Ask your money back if possible, perhaps you have bought a stolen iPhone.

If you can't get in touch with the seller, you can't use the iPhone.

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If you want to create a new Apple ID, see -> How to create a new Apple ID

How to create a new Apple ID – Apple Support (UK)

See also -> If you see a Could Not Create Apple ID or Could Not Set Up iCloud message

If you see a Could Not Create Apple ID or Could Not Set Up iCloud message - Apple Support

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What should you do?

If you have another iPhone, iPad, iPod touch, or Mac, you can try setting up iCloud with your Apple ID on that device. Sign in with your Apple ID, then follow the onscreen steps to start using iCloud. 

Or, if you already have an Apple ID set up with iCloud, sign in with that Apple ID instead. 

If you still need to set up iCloud with this Apple ID, contact Apple Support for help. 

Your Apple ID is the account you use to access Apple services like the App Store, Apple Music, iCloud, iMessage, FaceTime, and more. You sign in to all Apple services with this single Apple ID and password.

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You can’t merge multiple Apple ID accounts.

What happens when you use the same Apple ID & Password that you use to sign in here?

See the following Apple articles

Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)