Q: how to change my apple tv password

This is the password for the Apple ID which you use for iTunes, which may be the same one as you use elsewhere such as here or may be different. If you don't know the password you can reset it by going to https://iforgot.apple.com/password/verify/appleid.

Note that if this ID is also used for iCloud you must sign out of iCloud on all your devices before proceeding or you will get into difficulties. I don't knonw whether you can simply sign out on the Apple TV or whether you will have to reset it.