Click on this link to learn how to 'Change your Apple ID password' --> https://support.apple.com/HT201355
"After you change your Apple ID email address or password, you might need to update your sign-in information with any Apple services you're using" Click on this link to read "What to do after [or before] you change your Apple ID email address or password" --> https://support.apple.com/HT204071
- This is particularly important to do if you use the "Find My" feature. Disable Find My before changing your information, change the information, then reactivate it with the new information.
If you forgot your user or account name on Mac https://support.apple.com/guide/mac-help/if-you-forgot-your-user-or-account-name-mh35548/mac
If you can't reset your Mac login password --> https://support.apple.com/HT212190