How can I change/update the Apple ID & password associated with my old Mac Pro (Early 2009) running OS X El Capitan (Version 10.11.6)?

Hello webjock2,

Welcome to Apple Support Communities. What happens when you try to update apps on your Mac with OS X El Capitan (Version 10.11.6)? Are you getting any errors or specific behavior?

Are you using an Apple ID with two-factor authentication?

If so, here's how to use your Apple ID with two-factor authentication on older Apple devices:

1. Get the six-digit authentication code from one of your trusted devices associated with your Apple ID.
2. On your Mac, log in with your Apple ID and password, and add the six-digit code at the end of the password without any spaces.

You can also generate a verification code manually:

• In iOS, open Settings > [your account] > Password & Security and then tap Get Verification Code.
• In macOS, go to the iCloud preference pane, click the Security tab, and then click Get Verification Code.

For more information, please check this out:

Get a verification code and sign in with two-factor authentication - Apple Support

We hope this information is useful. Kind regards.