I am giving my Mac Pro to my wife. How do I change the default email address from mine to her's. Thank you

Presuming you and your wife are still amicable, the easiest thing to do is create a new account for her. MacOS is inherently a multi-User system, and creating only one Account (as many users do) does not change the multi-User nature in the slightest. it is ready for you to add her Account:

System Preferences > User & Groups

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If you are not amicable, do the steps you would do as if you were selling:

What to do before you sell, give away, or trade in your Mac - Apple Support

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