How can I reactivate an Apple ID (ex. Email address)

Do you want to change your Apple ID email address to the old email address? Note the restrictions in the article.

Apple ID Name Change.  * 

 What to do after you change your Apple ID or password - Apple Support  *  I suggest doing this before you change your Apple ID or password.

Or did you deactivate an Apple ID and you want to reactivate the ID? From the article below.

Can I reactivate my account?

Yes. If you request account deactivation, you'll receive a unique alphanumeric access code. When you're ready to reactivate your Apple ID, contact Apple Support, provide your access code, and we’ll help you reactivate your account. If you lose this code, we won't be able to help reactivate your account. 

Apple ID - How to deactivate your Apple ID account.