i've got an iphone frome my cousin that is blocked and they don't remember the password, how can i unblock it ?

If you forgot your passcode, you can use your Apple ID and password to erase and reset your device without a computer.

If you enter the wrong passcode on your iPhone Lock Screen too many times, an alert lets you know that your iPhone is disabled. (Learn what to do if you see "[Device] unavailable" or "Security Lockout" on your iPhone or iPad.)

Lock Screen Passcode or Screen Time Passcode.

Following this process is the only way out if your iPhone is disabled. The steps given in this link will help you reset the passcode -> If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Note: If you have forgotten the Screen Time Passcode then restoring your device from a backup after the passcode reset will return the Passcode back to your device. So, please do not restore from a backup.

iCloud uses Apple ID credentials.

If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support

If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.

It would be best if he goes through the process of resetting his password or doing account recovery to regain access to his password. Does your cousin have any other Apple devices such as an iPad, MacBook, iMac? If so he can reset his password in settings: If you forgot your Apple ID password - Apple Support

If he doesn't have any apple devices he can do account recovery. He needs to go to iforgot.apple.com and put his email in, and he'll need to know his trusted number associated with his apple ID. It doesn't need to be active or in use, just to verify... Please see this: How to use account recovery when you can’t reset your Apple ID password - Apple Support

If none of that works then he can do activation lock removal with proof of purchase: How to remove Activation Lock - Apple Support

If your cousin can't or isn't willing to do any of these steps then there's no other way to unlock it.

Good Luck!

You can't unlock the phone on your own. And since you aren't the original buyer with a receipt, showing the serial number, Apple won't unlock it for you either.

Your cousin is the only one who can unlock it. Here are a couple links you can share with your cousin, which they MUST do or you have a useless phone, which you can't use:

If you forgot your Apple ID password - Apple Support

If you forgot your Apple ID email address or phone number - Apple Support

If your cousin isn't able to do this, the only hope for the phone is for your cousin to present the original purchase receipt to Apple, with the iPhone's serial number clearly printed on the receipt. With that, Apple will help. Without it, Apple won't.

Then before you can set up the phone for yourself, the following link explains what needs to be done:

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

Honestly, whether this is worth the effort is up to you. iPhone 6 was released in 2014. It cannot run the latest iOS version and many apps will ultimately lose support on such an old out of date device. Personally, I'd give it back to your cousin and buy a newer, more modern iPhone.