How do I change my ICloud password

iCloud uses your Apple Account password. To change that, see instructions in Change your Apple Account password - Apple Support

If you have forgotten the password:

If you only have one Apple Account, it would be the same Account and password as you used to log into this forum. If you have another and have forgotten the Apple Account primary email or password, see the following:

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

or

If you forgot your Apple Account password - Apple Support