hello how can i factory reset my old locked iohone

If it is Passcode Lock, check this link...

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

If it is Activation Lock, check this link....

How to remove Activation Lock - Apple Support

If you do not remember the code for your iPhone 6, your data will be lost. But if you can remember the code for your Apple ID, then you can reset the phone. All data will be deleted, but you can at least use the phone.

Factory Reset your iPhone 6:

1) Turn off your phone

2) Connect your phone to a PC / Mac with your lightning Cable - the moment you plug in your iPhone, hold down the Home Button and hold it until you see a computer and cable icon. Your phone is now in recovery mode.

3) Select Restore on your computer and wait.

Your phone will be restored to factory settings. When you start it up you will be asked to enter the code for your Apple ID, this is Activation Lock.