How do I change my I-cloud password?

See this Apple article: If you forgot your Apple ID password

If you forgot your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

iCloud uses your Apple ID and password. If you only have one Apple ID, it would be the same ID and password as you used to log into this forum. If you have another and have forgotten the password, see the following: If you forgot your Apple ID password - Apple Support

or if you know the current password, see instructions in Change your Apple ID password - Apple Support