How can I change my Apple Account password?

You cannot change a password that you have forgotten. That would be a huge security failure if someone could change you password without knowing what it is. First you will need to gain access to your account:

If you forgot your Apple Account password - Apple Support

Then you can change the password:

Change your Apple Account password - Apple Support

See Change your Apple ID password Change your Apple Account password - Apple Support 

open setting , open iCloud and you see personal information tap personal information and you see chnge password option